[162] Child, C. M., _Senescence and Rejuvenescence_. Chicago, 1915.

[Illustration: FIG. 33]

The writer has observed more closely the transformation of an organ

into more undifferentiated material in _Campanularia_ (Fig. 33), a

hydroid.[163] This organism shows a remarkable stereotropism. Its

stolons attach themselves to solid bodies, and the stems appear on the

side of the stolon exactly opposite the point or area of contact with

the solid body. The stems grow, moreover, exactly at right angles to

the solid surface element to which the stolon is attached. If such

a stem be cut and put into a watch glass with sea water, it can be

observed that those polyps which do not fall off go through a series of

changes which make it appear as if the differentiated material of the

polyp were transformed into undifferentiated material. The tentacles

are first put together like the hairs of a camel’s-hair brush (Fig.

34), and gradually the whole fuses to a more or less shapeless mass

which flows back into the periderm (Fig. 35). It follows from this that

in this process certain solid constituents of the polyp, _e. g._, the

cell walls, must be liquefied. This undifferentiated material formed

from the polyp may afterward flow out again, giving rise to a stolon

or a polyp; to the former where it comes in contact with a solid body,

to the latter where it is surrounded by sea water. These observations

suggest the idea of reversibility of the process of differentiation

of organs and tissues, in certain forms at least. We have to imagine

that some of the cells or interstitial tissue is digested and that as a

consequence the organ loses its characteristic shape.

[163] Loeb, J., _Am. Jour. Physiol._, 1900, iv., 60.

[Illustration: FIG. 34]

[Illustration: FIG. 35]

Giard and Caullery have found that a regressive metamorphosis occurs

in Synascidians, and that the animals hibernate in this condition.

The muscles of the gills of these animals are decomposed into their

individual cells. The result is the formation of a parenchyma

which consists of single cells and of cell aggregates resembling a

morula.[164]

[164] The writer quotes this after Driesch.

Driesch,[165] experimenting on the regeneration of an Ascidian, found

that when he cut off the gills and siphons of the animal the portion

removed was able to regenerate a whole animal. The gill-piece excised

contained no heart, no intestine, and no stolon, and all these organs

were regenerated from the gills. In a number of cases the regeneration

took place by bud formation at the edge of the wound, but in other

cases the gills were transformed into an undifferentiated mass of

tissue from which the missing parts of the animals arose by budding and

new gills were formed.

[165] Driesch, H., _Arch. f. Entwcklngsmech._, 1902, xiv., 247.

It is probable that the two cases are only quantitatively different. In

both, autodigestion of certain cell constituents and possibly of whole

cells must take place in order to obtain material for the formation of

the lost part of the Ascidian. If an interstitial tissue is digested

it becomes a question of how much of this tissue undergoes hydrolysis.

If there is little destroyed the old shape of the gills remains, if

too much is digested the old gills become a shapeless mass in which a

certain number of the old cells are maintained and give rise to the new

animal by cell division. The material for the new organs must of course

be furnished from old cells which have been digested.

If regeneration takes place in pieces which take up no food the newly

formed organs must originate from material absorbed from cells of the

animal which are hydrolyzed and whose material serves as food for those

cells which grow. Very often this process of digestion takes place

without loss of the total form of the organ and is overlooked by the

pure morphologists. In _Campanularia_ also the process of collapse

described above is only apparent in a fraction of the cases as in

Driesch’s observations on _Clavellina_.[166] It is also possible that

the red and yellow entoderm cells which gather at the end where the

new polyp forms furnish the material which is utilized for the process

of growth of the cells from which the tentacles arise (with or without

giving off specific “hormones” besides).

[166] One author, Miss Thatcher, in trying to repeat these

observations, did not notice the total collapse of the tissues and

concluded that my observations must have been wrong. The writer is

fairly certain that his observations were correct.

6. We have mentioned the ideas concerning a design, or “entelechy,”

acting as a guide to the developing egg and have shown that this

revival of Platonic and Aristotelian philosophy in biology was due to a

misconception; namely, that the egg consisted of homogeneous material

which was to be differentiated into an organism. For this supernatural

task supernatural agencies seemed required. But we have seen that

the unfertilized egg is already differentiated in a way which makes

the further differentiation a natural affair. This idea of a quasi

superhuman intelligence presiding over the forces of the living is met

with in the field of regeneration, and here again it is based upon a

misconception. The lens of the eye is formed in the embryo from the

epithelium lying above the so-called optic cup (the primitive retina).

Where this retina touches the epithelium the latter begins to grow into

the cup, the ingrowing piece of epithelium is cut off and forms the

lens, which probably under the influence of substances secreted by the

optic cup becomes transparent. Certain animals like the salamander are

able to form a new lens when the old one has been removed by operation,

but the new lens is formed in an entirely different way; namely, from

the upper edge of the iris. G. Wolf, who observed this regeneration

used it to endow the organism with a knowledge of its needs; the idea

of a Platonic preconceived plan or an Aristotelian purpose suggested

itself. But it can be shown that the organism does in this case what

it is compelled to do by its physical and chemical structure.

Uhlenhuth[167] has shown by way of tissue culture that the cells of

the iris cannot grow and divide as long as they are full of pigment

granules as they normally are. When the fine superficial membrane

of the iris is torn the pigment granules fall out and the cells can

now grow and multiply. If the lens is taken out of the eye of the

salamander the fine membrane of the iris is torn and the pigment cells

at the edge (especially the upper edge) lose their pigment granules

which fall down on account of their specific gravity. As soon as this

happens the cells will proliferate. A spherical mass of cells is formed

which become transparent and which will cease to grow as soon as they

reach a certain size. The unanswered question is: Why does the mass of

cells become transparent so that it can serve as a lens? The answer is

that young cells when put into the optic cup always become transparent

no matter what their origin; it looks as if this were due to a chemical

influence exercised by the optic cup or by the liquid it contains.

Lewis has shown that when the optic cup is transplanted into any other

place under the epithelium of a larva of a frog the epithelium will

always grow into the cup where the latter comes in contact with the

epithelium; and that the ingrowing part will always become transparent.

This leaves us then with one puzzle still: Why is the growth of the

lens limited? The limitation in the growth of organs is one of the most

important problems in growth and organ formation, though unfortunately

our knowledge of this topic is inadequate.

[167] Not yet published.

7. The botanist J. Sachs was the first to definitely state that in

each species the ultimate size of a cell is a constant, and that two

individuals of the same species but of different size differ in regard

to the number, but not in regard to the size of their cells.[168]

Amelung, a pupil of Sachs, determined the correctness of Sachs’s

theory by actual counts. Sachs, in addition, recognized that wherever

there were large masses of protoplasm, _e. g._, in siphoneæ and other

cœloblasts, many nuclei were scattered throughout the protoplasm. He

inferred from this that “each nucleus is only able to gather around

itself and control a limited mass of protoplasm.”[169] He points

out that in the case of the animal egg the reserve material--fat

granules, proteins, and carbohydrates--are partly transformed into the

chromatin substances of the nuclei, and that the cell division of the

egg results in the cells reaching a final size in which each nucleus

has gathered around itself that mass of protoplasm which it is able

to control. Morgan[170] and Driesch[171] tested and confirmed the

idea of Sachs for the eggs of Echinoderms. We stated in the previous

chapter that Driesch produced artificially larvæ of sea urchins of

one-eighth, one-fourth, and one-half their normal size by isolating a

single cleavage cell in one of the first stages of segmentation of the

fertilized sea-urchin egg. He counted in each of the dwarf gastrulæ

resulting from these partial eggs the number of mesenchyme cells

and found that the larvæ from a one-half blastomere possessed only

one-half, those from a one-fourth blastomere only one-fourth, and those

from a one-eighth blastomere only one-eighth of the number of cells

which a normal larva developing from a whole egg possessed. Moreover,

he could show that when two eggs were caused to fuse so as to produce

a single larva of double size, the gastrulæ of such larvæ had twice

the number of mesenchyme cells. Driesch drew the conclusion from his

observations that each morphogenetic process in an egg reaches its

natural end when the cells formed in the process have reached their

final size.

[168] v. Sachs, J., “Physiologische Notizen,” vi., _Flora_, 1893.

[169] _Ibid._, ix., 425, _Flora_, 1895.

[170] Morgan, T. H., _Arch. f. Entwcklngsmech._, 1895, ii., 81; 1901,

xiii., 416; 1903, xvi., 117.

[171] Driesch, H., _Arch. f. Entwcklngsmech._, 1898, vi., 198; 1900,

x., 361.

Since each daughter nucleus of a dividing blastomere has the same

number of chromosomes as the original nucleus of the egg, it is clear

that in a normally fertilized egg each nucleus has twice the mass of

chromosomes that is contained in the nucleus of a merogonic egg, _i. e._,

an enucleated fragment of protoplasm into which a spermatozoön

has entered and which is able to develop. Such a fragment has only

the sperm nucleus. This phenomenon of merogony was discovered by

Boveri and was elaborated by Delage.[172] Boveri, in comparing the

final size of the cells in normal and merogonic eggs after the cell

divisions had come to a standstill, found that this size is always

in proportion to the original mass of the chromatin contained in the

egg; the cells of the merogonic embryo, _e. g._, the mesenchyme cells,

are only half the size of the same cells in the normally fertilized

embryo. Driesch furnished a further proof of Boveri’s law, that the

final ratio of the mass of the chromatin substance in a nucleus to the

mass of protoplasm is a constant in a given species. Driesch compared

the size of the mesenchyme cells in a sea-urchin embryo produced by

artificial parthenogenesis with those of a normally fertilized egg and

found them half of the size of the latter. When the fertilized eggs and

the parthenogenetic eggs are equal in size from the start,--which is

practically the case if eggs of the same female are used,--the process

of the formation of mesenchyme cells comes to a standstill when their

number in the normally fertilized eggs is half as large as the final

number in the parthenogenetic egg.[173] Boveri’s results as well as

those of Driesch were obtained by counting the cells formed by eggs of

equal size and not by simply measuring the size of the cells. It is

most remarkable that certain apparent exceptions to Boveri’s law which

Driesch has actually found had been predicted by Boveri.

[172] Delage, Y., _Arch. Zoöl. expér._, 1899, vii., 383.

[173] Driesch, H., _Arch. f. Entwcklngsmech._, 1905, xix., 648.

These facts show that the growth of an organ comes to a standstill when

a certain size is reached or a certain number of cells are formed. We

cannot yet state why this should be, but we are able to add that the

formation of a lens of normal size in the regeneration of the eye is

in harmony with the phenomena in the embryo. There seems therefore

no reason for stating that the regeneration of the lens cannot be

explained on a purely physicochemical basis. The only justification for

such a statement on the part of Wolf is that he was not in possession

of the more complete set of facts now available through the work of

Fischel and Uhlenhuth.

The healing of a wound is a process essentially similar to the

regeneration of the lens. Normally the cells which begin to proliferate

after a wound is made in the skin lie dormant, inasmuch as they neither

grow nor divide. When a wound is made certain layers of epidermal

cells undergo rapid cell division. Leo Loeb[174] has studied this case

extensively. He found that if the skin is removed anywhere, epidermis

cells from the wound edge creep upon the denuded spot and form a

covering. This may be a tropism (stereotropism) or it may be a mere

surface tension phenomenon. Next a rapid process of cell division

begins in the cells adjacent to the wound these cells having been

heretofore dormant. He is inclined to attribute this increase in

the rate of cell division to the stretching of the epithelial cells,

and he is supported in this reasoning by the observation that the

larger the wound the more rapid the process of healing.[175] During

wound healing the mitoses first increase markedly in the old epithelium.

With the closure of the wound a sudden fall in the mitoses takes

place. The closure of the wound causes an increase in the number of

epithelial rows over the defect. This increase is therefore reached at

an earlier period in the larger wound since the process of mitosis is

more rapid here. Leo Loeb thinks that the pressure of the epithelial

cells upon each other leads to a rapid diminution in the mitotic

proliferation.[176]

[174] Loeb, Leo, _Arch. f. Entwcklngsmech._, 1898, vi., 297.

[175] Spain, K. C., and Loeb, Leo, _Jour. Exper. Med._, 1916, xxiii.,

107; Loeb, L., and Addison, W. H. F., _Arch. f. Entwcklngsmech._, 1911,

xxxii., 44; 1913, xxxvii., 635.

[176] The excessive formation of epithelial cells in the healing of

wounds has led the older pathologists to the generalization that if

something is removed in the body an excessive compensation will take

place. The formation of antibodies has even been explained on this

basis by Weiggert and Ehrlich in their side-chain theory. As a matter

of fact, this generalization is entirely incorrect and in regeneration

of starfish, actinians, flatworms, annelids, and possibly in all

forms the reverse is true; _e. g._, if we cut off the anterior half

of the body in _Cerianthus_ less is reproduced than was cut away

namely only tentacles and the mouth, but not the missing piece of the

body. Weiggert’s conception of regeneration was probably based on the

phenomenon of the healing of wounds, but the excessive epithelium

formation in this case is not the expression of a general law of

regeneration but of the peculiar mechanical conditions which lead to

mitoses. It would be a very strange coincidence indeed if a theory of

antibody formation based on such an erroneous generalization should be

correct.

Should it be possible that this is more generally the case, _e. g._,

also in the lens after it has reached a certain size? The conditions

limiting growth require further investigation.

It is hardly necessary to point out that in these cases we are

seemingly dealing with cases of the inhibition of growth which cannot

be explained by the tyranny of the whole over the parts, and that there

must be conditions at work other than the mere flow of substances which

can cause a cessation of growth. This can be illustrated by certain

observations on the egg.

8. The history of the egg shows a reversible condition of rest and

of activity. The primordial egg cell multiplies actively until a

large number of eggs are formed in the ovary which may reach into the

millions in the case of sea urchins or certain annelids. These cell

divisions then stop and the egg goes into the resting stage in which

it deposits the reserve material for the development of the embryo.

From this condition it can only be called into activity again by the

spermatozoön or the agencies of artificial parthenogenesis.

It seemed of interest to find out whether or not the development of

the egg may be reversed once more after it has been activated. From

all that has been said in the chapter on artificial parthenogenesis,

such a reversal should take place in the cortical layer. The result of

these experiments seems to be that if a complete destruction or change

in the cortical layer has once taken place--such as that caused by

the entrance of a spermatozoön into the egg--no reversal is possible;

although the development of the fertilized egg may be suppressed

for a long time by either low temperature or lack of oxygen, or, in

the case of seeds and spores, by lack of water. But as soon as the

conditions for the chemical reactions in the egg are normal again, the

development may go on unless the egg has suffered by the methods used

to prevent development or by the long duration of the suppression. With

an incomplete destruction of the cortical layer both development as

well as reversal of development are possible. Thus the writer has shown

that in the egg of _Arbacia_ the effect of the cortical alteration

of the egg induced by the butyric acid treatment or by the treatment

with bases can be reversed. When unfertilized eggs of _Arbacia_ are

put for from two to five minutes into 50 c.c. sea water + 2.0 c.c.

N/10 butyric acid they will all form a gelatinous, somewhat atypical

fertilization membrane; when put back into normal sea water all will

perish in a few hours unless they are submitted to the short treatment

with a hypertonic solution mentioned in the previous chapter, while if

submitted to this treatment they will develop. If, however, these eggs

are transferred from the butyric acid sea water not into normal sea

water but into sea water containing some NaCN (10 drops of 1/10 per

cent. NaCN or KCN in 50 c.c. sea water), and if they remain here for

some time (_e. g._ overnight) they will not perish when subsequently

transferred back to normal sea water. Such eggs will develop when

fertilized with sperm. The activating effect of the membrane formation

has, therefore, been reversed and the eggs have gone back into the

resting stage.[177] Wasteneys has found that the rate of oxidation

which was raised considerably by the artificial membrane formation

goes back to the value characteristic for the resting eggs after the

reversal of their developmental tendency.[178] Similar results were

obtained in eggs activated with NH₄OH. It appears from this as though

the change in the cortical layer which leads to the development of the

egg and the increase in the rate of oxidations were reversible in the

egg of _Arbacia_.[179]

[177] Loeb, _Arch. f. Entwcklngsmech._, 1914, xxxviii., 277.

[178] Wasteneys, H., _Jour. Biol. Chem._, 1916, xxiv., 281.

[179] F. Lillie thinks that the KCN in this experiment merely inhibits

the change of the cortical layer necessary for development. This is

contradicted by two facts: first, the writer has shown in 1906 that KCN

does not inhibit the membrane formation, and, second, the eggs will not

return to the resting stage when put back into sea water too soon; in

that case they will disintegrate. This shows that in the KCN something

more happens than the mere block to disintegration.

The writer had previously noticed that eggs of _Strongylocentrotus

purpuratus_, which had been treated for two hours with hypertonic

sea water, not infrequently began to divide into two, four, or eight

cells (and sometimes more) and then went back into the resting state

(except that they possessed the second factor required for development

as stated in Chapter V). It may be remarked incidentally that such

eggs at the time of cell division contained the centrosomes and

astrospheres, and yet went back into a resting state, thus showing that

the centrosomes are only transitory organs or organs which are only

active under certain conditions. It is quite possible that in these

phenomena of reversal not the whole of the cortical layer has undergone

alteration.

The writer must leave it undecided whether the changes from the resting

to the active state in body cells can also be explained in analogy with

these experiments.

9. In the formation of the lens we have already noticed an instance

where the adjacent organ influences growth inasmuch as the optic

cup controlled the formation of the lens. Such influences are quite

commonly observed. A piece of _Tubularia_ when cut out from a stem and

suspended in water will regenerate at the aboral pole not a stolon but

a polyp, so that we have an animal terminating at both ends of its body

in a head. The writer called such cases in which an organ is replaced

by an organ of a different kind heteromorphosis.

Contact with a solid body favours the formation of stolons. Fig. 36

shows a piece of a stem of _Pennaria_ another hydroid, which was lying

on the bottom of an aquarium and which formed stolons at both ends _a_

and _b_. In _Margelis_, another hydroid, the writer observed that

without any operation the apical ends of branches which were in contact

with solid bodies continued to grow as stolons, while those surrounded

by sea water continued to grow as stems.

[Illustration: FIG. 36]

Herbst discovered a very interesting form of heteromorphosis in certain

crustaceans; namely, that in the place of an eye which was cut off, an

entirely different organ could be formed, namely, an antenna. He showed

that the experimenter has it in his power to determine whether the

crustacean shall regenerate an eye or an antenna in place of the eye.

The latter will take place when the optic ganglion is removed with the

eye, the former when it is not removed. These experiments were carried

out successfully on _Palæmon_, _Palæmonetes_, _Sicyonia_, _Palinurus_,

and other crustaceans.

The influence of gravitation is very familiar in plants; in stems of

_Bryophyllum_ placed horizontally the roots usually come out from the

lower end of the callus. Such phenomena are not often found in animals

but they exist here too as the following observation shows.

[Illustration: FIG. 37]

If we cut a piece _a b_ (Fig. 37), from the stem _s s_ of _Antennularia

antennina_ (Fig. 38), a hydroid, and put it into the water in a

horizontal position, new stems _c d_ (Fig. 37) may arise on its upper

side. The small branches on the under side of the old stem _a b_

begin suddenly to grow vertically downward.[180] In appearance and

function these downward-growing elements are entirely different from

the branches of the normal _Antennularia_; they are roots. In order

to understand better the transformation which thus occurs in these

branches, it may be stated that under normal conditions they have a

limited growth (see Fig. 38), are directed upward, and have polyps on

their upper side. The parts which grow down (Fig. 37) have no polyps,

but attach themselves like true roots to solid bodies. Thus the

changed position of the stem alone, without any operation, suffices

to transform the lateral branches, whose growth is limited, into

roots with unlimited growth. The lateral branches on the upper side

of the stem do not undergo such a transformation into roots except in

the immediate surroundings of the place where a new stem arises. It

seems that the formation of a new stem also causes an excessive growth

of roots, possibly because the formation of new branches causes the

removal of substances which naturally inhibit the formation of roots.

If a piece from the stem be put vertically into the water with top

downward, the uppermost point may continue to grow as a stem, while the

lowest point may give rise to roots. In this case, therefore, a change

in the orientation of organs has the effect of changing the character

of organs.

[180] Loeb, J., Untersuchungen zur physiologischen Morphologie der

Tiere. II. Organbildung und Wachsthum. Würzburg, 1892.

[Illustration: FIG. 38]

There are only two ways by which we can account for these influences of

gravitation. Either certain substances flow to the lowest level and

collecting there induce growth and possibly changes in the character

of growth (as in _Antennularia_) or if the cells have elements of

different specific gravity the relative position of these elements may

possibly change and influence in this way the conditions for growth.

The influence of gravitation as well as of contact upon life phenomena

are at present little understood.

In all these cases of heteromorphosis the original form is not

restored. It is needless to say that they are incompatible with the

theory of natural selection.

The reader will have noticed that in this chapter one term has

not been mentioned which is commonly met with in the literature,

namely the “wound stimulus.” As the writer had indicated in a former

publication,[181] the word “stimulus” is generally used to disguise

our ignorance of (and also our lack of interest in) the causes which

underlie the phenomena which we investigate. Regeneration very often

does not take place near the wound but at some distance from it. But

even when the regeneration takes place at the edge of the wound the

latter only serves to create conditions for regeneration, and these

conditions cannot be expressed by the word “stimulus.”

[181] Loeb, J., _Die chemische Entwicklungserregung des tierischen

Eies_. Berlin, 1909.

While our knowledge of the rôle of the whole in regeneration is

incomplete in a great many details it seems that the known facts

warrant the statement that the phenomena of regeneration belong as much

to the domain of determinism as those of any of the partial phenomena

of physiology.

CHAPTER VIII

DETERMINATION OF SEX, SECONDARY SEXUAL CHARACTERS, AND SEXUAL INSTINCTS

_I. The Cytological Basis of Sex Determination_

1. It is a general fact that both sexes appear in approximately equal

numbers, provided a sufficiently large number of cases are examined.

This fact has furnished the clue for the discovery of the mechanism

which determines the relative number of the two sexes. The honour

of having pointed the way to the solution of the problem belongs

to McClung.[182] It has been known that certain insects, _e. g._,

Hemiptera and Orthoptera, possess two kinds of spermatozoa but only one

kind of eggs. The two kinds of spermatozoa differ in regard to a single

chromosome, which is either lacking or different in one-half of the

spermatozoa.

[182] McClung, C. E., “The Accessory Chromosome--Sex Determinant?”

_Biol. Bull._, 1902, iii., 43.

The first one to recognize the existence of two kinds of spermatozoa

was Henking, who stated that in _Pyrrhocoris_ (a Hemipteran) one-half

of the spermatozoa of each male possessed a nucleolus, while in

the other half it was lacking. Montgomery afterward showed that

Henking’s nucleolus was an accessory chromosome. McClung was the

first to recognize the importance of this fact for the problem of sex

determination. He observed an accessory chromosome in one-half of the

spermatozoa of two forms of Orthoptera, _Brachystola_ and _Hippiscus_,

and reached the following conclusion:

A most significant fact, and one upon which almost all

investigators are united in opinion, is that the element is

apportioned to but one-half of the spermatozoa. Assuming it to

be true that the chromatin is the important part of the cell

in the matter of heredity, then it follows that we have two

kinds of spermatozoa that differ from each other in a vital

matter. We expect, therefore, to find in the offspring two sorts

of individuals in approximately equal numbers, under normal

conditions, that exhibit marked differences in structure. A careful

consideration will suggest that nothing but sexual characters thus

divides the members of a species into two well-defined groups,

and we are logically forced to the conclusion that the peculiar

chromosome has some bearing upon the arrangement.

N. M. Stevens and E. B. Wilson[183] have not only proved the

correctness of this idea for a number of animals but have laid the

foundation of our present knowledge of the subject. Wilson showed that

in those cases where there are two types of spermatozoa, one with and

one without an accessory or as it is now called an X chromosome, all

the cells of the female have one chromosome more than the cells of the

male. From this he concludes correctly that in such species a female is

produced when the egg is fertilized by a spermatozoön containing an X

chromosome, while a male is produced when a spermatozoön without an X

chromosome enters the egg.

[183] Wilson, E. B., “Studies on Chromosomes,” _Jour. Exper. Zoöl._,

1905, ii., 371, 507; 1906, iii., 1; 1909, vi., 69, 147; 1910, ix.,

53; 1912, xiii., 345. “Croonian Lecture,” 1914, _Proc. Roy. Soc._, B.

lxxxviii., 333.

Such a form is _Protenor_, one of the Hemiptera. Wilson made sure

that all the eggs are alike in the number of chromosomes, each

egg containing an X chromosome in addition to the six chromosomes

characteristic of the species _Protenor_. There are two types

of spermatozoa in equal numbers in this species, each with six

chromosomes, but one with, the other without, an X chromosome. The

two possible chromosome combinations between egg and spermatozoa are

therefore as follows (see the diagrammatic Fig. 39):

_Egg_ _Spermatozoön_ _Result_

(1) 6 + X + 6 = 12 + X = Male

(2) 6 + X + 6 + X = 12 + 2X = Female

The egg which receives a spermatozoön without an X chromosome has after

fertilization 12+X chromosomes and develops into a male; while the egg

into which a spermatozoön with an X chromosome enters gives rise to

a female. Since all the body cells arise from the fertilized egg by

nuclear division and the chromosomes remain constant in number in all

cells, the consequence is that all the cells of a female _Protenor_

have two X chromosomes; while all the cells of a male _Protenor_ have

only one X chromosome.

[Illustration: FIG. 39]

The chromosome situation in _Protenor_ is a somewhat extreme case,

inasmuch as one X chromosome is entirely lacking in the male. In other

forms of Hemiptera, _e. g._, _Lygæus_, there are also two types of

spermatozoa appearing in equal numbers differing in regard to the

X chromosome, but here it is only a difference in size; one-half of

the spermatozoa having a large X chromosome, the other half instead

a smaller chromosome. Calling this latter the Y chromosome, the sex

determination in this form is as follows: leaving aside the chromosomes

which are equal in both egg and spermatozoön we may say that there is

one type of egg containing one large X chromosome; there are two types

of spermatozoa in equal numbers, one possessing a large X chromosome,

the other possessing a small Y chromosome. Wilson showed by a study of

the chromosomes in males and females that when one of the spermatozoa

containing a large X chromosome enters the egg, the egg will develop

into a female; while when one of the spermatozoa containing a small Y

chromosome enters it will give rise to a male. Leaving aside the common

chromosomes of both sexes, a fertilized egg containing XX gives rise

to a female, while one containing XY gives rise to a male. There is

in this case as in that of _Protenor_ a preponderance of chromosome

material in the female, but this quantitative difference is not

essential for the determination of sex, since in some species the Y

chromosome may be as large as the X chromosome.

The main fact is that the female cells have the chromatin composition

XX, the male cells the composition XY, where Y is apparently

qualitatively different and often, but not necessarily, smaller than X,

or entirely lacking.

It may be mentioned in passing that indirect evidence exists indicating

that in man there are also two kinds of spermatozoa and one kind of

egg, and that sex depends on whether a male determining or a female

determining spermatozoön enters the egg.

2. This mode of sex determination holds only for those animals in which

there is one type of egg and two types of spermatozoa. Experimental

evidence furnished first by Doncaster in 1908 on a moth, _Abraxas_,

indicated that a number of other forms exists in which matters are

reversed, inasmuch as there are two types of eggs and one type of

spermatozoa. This condition of affairs exists not only in the moth

_Abraxas_, but also in the fowl as shown by Pearl. In these forms it

is assumed that all the spermatozoa have one sex chromosome X, while

there are two types of eggs, one possessing the sex chromosome X,

the other possessing Y. When a spermatozoön enters an egg with an X

chromosome, the egg will give rise to a male, while if it enters a Y

egg, a female will arise. The evidence pointing toward this result

is chiefly contained in experiments on sex-limited or more correctly

sex-linked heredity; _i. e._, a form of heredity which follows the

sex in a peculiar way. Thus colour-blindness is a case of sex-linked

inheritance, since this abnormality appears overwhelmingly in the male

offspring of a colour-blind person. Doncaster crossed two varieties of

_Abraxas_ differing in one character which was sex-linked, and the

results of his crossings indicated that in this form there are two

types of eggs and one type of spermatozoa.[184]

[184] Doncaster, L., _The Determination of Sex_. Cambridge, 1914.

These observations on sex-linked heredity confirm the idea that the

sex chromosomes determine the sex. The most extensive and conclusive

experiments along this line are those by Morgan on the fruit fly

_Drosophila_. In this form there are two kinds of spermatozoa and one

kind of eggs; the egg has one X chromosome, while one-half of the

spermatozoa has an X the other a Y chromosome; the entrance of the

latter into an egg gives rise to a male, of the former to a female.

While the eyes of the wild fruit fly _Drosophila ampelophila_ are red,

Morgan[185] noticed in one of his cultures a male that had white eyes.

This white-eyed male was mated to a red-eyed female. The offspring, the

F₁ generation, were all red eyed, males as well as females. These were

inbred and now gave in the F₂ generation the following three types of

offspring:

[185] Morgan, T. H., _Heredity and Sex_. New York, 1913.

(1) 50 per cent. females, all with red eyes.

(2) 50 per cent. males { 25 per cent. with red eyes.

{ 25 per cent. with white eyes.

The character white eye was therefore transmitted only to half the

grandsons; it was a sex-linked character. It is known from a study of

the pedigrees of colour-blind individuals that if the corresponding

experiment had been carried out with them, instead of with white-eyed

flies, the same proportions of normal and colour-blind would have been

found: namely, normal colour vision in the F₁ generation, in both males

and females, and half of the males of the F₂ generation colour-blind,

the other half and all the females with normal vision. Of course, in

man, intermarriage between two different F₁ strains would have been

required in place of the inbreeding of the F₁ generation, which took

place in Morgan’s experiments. Morgan interprets his experiments as

follows. The normal red-eyed _Drosophila_ has one kind of eggs, each

possessing one X chromosome. This X chromosome has also the factor

for the development of red-eye pigment. The white-eyed male has two

kinds of spermatozoa, one with an X chromosome, the other with a

Y chromosome, both lacking the factor for red-eye pigment. If we

designate the X chromosome with the factor for red-eye pigment by =X= and

the X and Y chromosomes lacking the factor for redness with X and Y the

following combinations must result if we cross a normal red-eyed female

with a white-eyed male:

_Eggs_ _Sperm_ _Result_

=X= X =X=X red-eyed female

=X= Y =X=Y red-eyed male

It is obvious that all the offspring of the first generation (the

F₁ generation) must be red eyed, since all the eggs have one =X=

chromosome with the factor for red. According to the results obtained

from cytological studies which will be explained in the next chapter,

the females with the chromatin constitution =X=X will form two types

of eggs in equal numbers: namely, eggs with an =X= and eggs with an X,

_i. e._, all eggs have one X chromosome, but in fifty per cent. of the

eggs the =X= has the factor for red, in fifty per cent. this factor is

lacking (X). The males having the chromosome constitution =X=Y form

two types of spermatozoa, one with an =X= possessing the factor for

red pigment and one, the Y chromosomes, lacking this factor. If inbred

the next F₂ generation will give rise to the following four types of

offspring: (1) =X==X=, (2) =X=X, (3) =X=Y, (4) XY, all four types in

equal numbers.

(1) and (2) give females, both red eyed, since both contain a

red-factored =X= chromosome. (3) and (4) give males, (3) giving rise to

red-eyed males, since it contains a red-factored =X= chromosome, (4)

producing males with white eyes since this X chromosome is lacking the

factor for red eyes. Since all four combinations must appear in equal

numbers (provided the experimental material is ample enough, which was

the case in these experiments), in the F₂ generation both males and

females should have red eyes and in the F₂ generation all the females

should have red eyes and half of the males should have red, half white

eyes. These results were obtained.

The experiments were carried further. No white-eyed females had

appeared thus far. On the same assumptions of the relation of the =X=,

X, and Y chromosomes to the heredity of sex as well as to eye colour it

was possible to predict under what conditions and in which proportions

white-eyed females should arise. Thus if a red-eyed female of the F₂

generation (a cross between white-eyed male and normal female) be

mated with a white-eyed male the result should be an equal number of

white-eyed males and white-eyed females if the chromosome theory of sex

determination were correct. The reasoning would be as follows:

The red-eyed female, having the chromosome constitution =X=X should

form two kinds of eggs in equal numbers with the constitution =X= and

X; the white-eyed male having the chromosome constitution XY should

form two kinds of spermatozoa X and Y. The following four types of

individuals must then be produced in equal numbers:

(1) =X=X, (2) XX, (3) =X=Y, and (4) XY.

In this case (2) must give rise to white-eyed females and (4) to

white-eyed males, while (1) must give rise to red-eyed females and (3)

to red-eyed males. Hence white-eyed males and females and red-eyed

males and females are to be expected in this case in equal numbers, and

this was actually observed.

The numerical agreement in this and the other experiments between the

expected and observed result cannot well be an accident. The fact that

the inheritance of sex-linked characters in man follows the same laws

as in _Drosophila_ is a strong argument in favour of the assumption

that in man, also, sex is determined by two kinds of spermatozoa.

Morgan and his students discovered no less than thirty-six sex-linked

characters in _Drosophila_, and each behaved in a similar way to the

red and white eye colour in regard to sex-linked inheritance, so that

the chromosome theory of sex determination rests on a safe basis.

That sex is merely determined by the number of X chromosomes, not by

the Y chromosome, is proved by the facts that the Y chromosome may be

completely absent as in _Protenor_ and that Bridges[186] has found a

type of female _Drosophila_ with a chromosome formula XXY whose sex was

not affected by the supernumerary Y.

[186] Bridges, C. B., _Genetics_, 1916, i., 1.

3. On the basis of all these experiments and theories it is

comparatively easy to explain a number of phenomena concerning sex

ratios which before had been very puzzling. In bees it had been shown

many years ago by Dzierzon that the males develop from unfertilized

eggs while the females, queens and workers, develop from fertilized

eggs. This is intelligible on the assumption that the unfertilized egg

contains only one X chromosome while the spermatozoön carries into the

egg the second X chromosome. But if the male bee produces two types

of spermatozoa we should expect that only one-half of the fertilized

eggs should be females, the other half males. But it happens that of

the two types of spermatozoa only one is formed since in one of the

cell divisions which lead to the formation of spermatozoa one viable

spermatozoön only is formed while the other one perishes. It is,

therefore, quite possible that it is the female-producing spermatozoön

which survives while the male-producing spermatozoön dies.

It is occasionally observed that an insect shows one sex on one side

of its body and the opposite sex on the other side. Boveri suggested

that this phenomenon of gynandromorphism is due to the fact that the

spermatozoön for some unknown reason does not fuse with the egg nucleus

until after the egg has undergone its first cell division. In this

case it fuses with the nucleus of one of the two cells into which

the egg divides (or in some cases even one of the later cells?). As

a consequence the one-half of the embryo which arises from the cell

which was not fertilized would have only one X chromosome and in a case

like the bee would develop parthenogenetically, while the other half

of the body, developing from the cell into which a spermatozoön has

penetrated, would be fertilized. The latter half of the body would be

female, the former male. In his last paper before his untimely death,

Boveri has given proof for the correctness of this interpretation as

far as gynandromorphism in the bee is concerned.[187]

[187] Boveri, Th., _Arch. f. Entwcklngsmech._, 1915, xlii., 264.

It seems to be generally true that where sexual reproduction leads

only to the formation of females the case finds its explanation in

the fact that the male-producing spermatozoa perish and only the

female-producing spermatozoa survive. Such an observation was made by

Morgan on a certain species of phylloxerans.

The slight preponderance in the number of one sex which is occasionally

found--an excess of six per cent. males over females in the human

race--may well find its explanation on the assumption of a slightly

greater mortality of the female-determining spermatozoa.

In certain forms parthenogenetic and sexual reproduction may alternate

in a cycle, _e. g._, in plant lice, _Daphnia_, and rotifers. In plant

lice it has been observed for a long time that when the plant is

normal and the weather warm the aphides remain wingless, reproduce

parthenogenetically, and only females exist, and this may last for

years and for more than fifty generations; but that when the plant is

allowed to dry out both sexes appear.

Here we are dealing with a limited determination of sex inasmuch as the

experimenter has it in his power to prevent or allow the production of

males. The facts do not in all probability contradict the statements

made concerning the rôle of the X chromosomes in the determination

of sex. We have seen that where sex is determined by two types of

spermatozoa one type of eggs is produced which possesses only one X

chromosome. Such eggs might produce males if not fertilized (as they

do in bees), but they cannot produce females because for that purpose

they must have two X chromosomes. It has been shown for certain

cases, and it may be true generally, that if eggs of this type give

rise to parthenogenetic females they may do so because they have for

some reason two X chromosomes. Usually such an egg loses one of the X

chromosomes in a process of nuclear division (the so-called reduction

division) which usually precedes fertilization. If this reduction

division is omitted the egg has two X chromosomes and if such an egg

develops parthenogenetically it gives rise to a female. These cases do

not, therefore, contradict the connection between X chromosomes and

sex determination established by cytological observations and breeding

experiments, on the contrary, they confirm it. The question remains:

How can external conditions bring it about that the reduction division

is omitted? To this question no definite answer can be given at present.

We may in passing mention the well-known observation that twins which

originate from the same egg always have the same sex; while twins

arising from different eggs show the usual variation as to sex. Twins

coming from one egg have the same chorion and can thereby be diagnosed

as such. They can be produced as we have stated in Chapter V by a

separation of the first two cleavage cells of the egg, each one giving

rise to a full embryo. It harmonizes with all that has been said above

that the sex of two such individuals must be the same since they have

the same number of X chromosomes, the latter being determined in the

human race by the nature of the spermatozoön which enters the egg.

4. While thus far all the facts agree with the dominating influence

of certain chromosomes upon sex determination, one group of facts has

not yet been explained: namely, hermaphroditism. By hermaphroditism is

meant the existence of complete and separate sets of female and male

gonads in the same individual. This condition exists regularly not

only in definite groups of animals, _e. g._, certain snails, leeches,

tape-worms, but also, as everybody knows, in flowering plants. While

in some forms both kinds of sex cells, male and female, are formed

and mature simultaneously, as, _e. g._, in the Ascidian _Ciona_ (see

Chapter IV), in others they are formed successively, very often the

spermatozoa appearing first (protandric hermaphroditism). In the long

tapeworm _Tænia_ each ring has testes and ovaries, but the young rings

are only male while in the older rings the testes disappear and the

ovaries are formed. The same ring is in succession male and female.

How can we reconcile the facts of hermaphroditism with the chromosome

theory of sex determination? _Rhabdonema nigrovenosum_, a parasite

living in the lungs of the frog, is hermaphroditic, but its eggs

produce not a hermaphroditic generation but one with the two separate

sexes; this generation is not parasitic and lives in the soil. The

generation produced by these separate males and females gives rise

again to a hermaphrodite which migrates into the lungs of the frogs.

According to Boveri and Schleip[188] the cells of the hermaphrodite

have twelve chromosomes. It produces two types of spermatozoa with

six and five chromosomes respectively (one-half of the cells losing

one chromosome which is left at the line of cleavage between the two

cells); and one type with six chromosomes. In this way separate males

and females are produced by the hermaphrodite, females with twelve and

males with eleven chromosomes.

[188] Boveri, Th., _Verhand. d. phys.-med. Gesellsch._ Würzburg, 1911,

xli., 85. Schleip, W., _Ber. d. naturf. Gesellsch._, Freiburg i. Br.,

1911, xix.

The males produce again two kinds of spermatozoa, male and female

producing, but the male-producing spermatozoa become functionless.

This fusion of the other spermatozoön containing six chromosomes with

an egg having six chromosomes leads again to the formation of the

hermaphrodite with twelve chromosomes. It is obvious that in this case

the cause for the hermaphroditism is not disclosed. If chromosomes

have anything to do with hermaphroditism there must be an undiscovered

element in the chromosomes which may explain why the female as well

as the hermaphrodite have the same chromosome constitution; or we are

forced to look for another determinant outside the X chromosomes or

the chromosomes altogether. This seems to be the only cytological work

on the problem of hermaphroditism. Experimental work has been begun by

Correns[189] and by Shull on the determination of hermaphroditism in

plants but lack of space forbids us to give details.

[189] Correns, C., _Biol. Centralbl._, 1916, xxxvi., 12.

_II. The Physiological Basis of Sex Determination_

5. As stated at the beginning of this chapter, the chromosome theory

of sex determination explained only one feature of the problem,

namely, the relative numbers in which both sexes or only one sex, as

the case may be, are produced; and in this respect the evidence is so

complete that we must accept it. But with all this, the problem of

sex determination is not exhausted, since a physiological solution of

the problem of sex determination demands an account of how the sex

chromosomes can induce the formation not only of ovaries and testes

but also of the other sex characters. For the solution of this problem

biology will have to depend largely on experiments in which it is

possible to influence the formation of sex characters and of the sex

glands themselves.

The most striking observations in this direction were made by Baltzer

on a marine worm, _Bonellia_. In this animal the two sexes are very

different, the male being a tiny parasite, a few millimetres in length,

which spends its life in the uterus of the female, whose size is about

five centimetres. A female carries as a rule several and often a large

number of the male parasites in its uterus, which indicates that the

males prevail numerically. The fertilized eggs of the animals are

laid in the sea water where the larvæ hatch. At the time of hatching

all larvæ are alike. The differentiation of the larvæ into the dwarf

males and the giant females can be determined at will. The larvæ have

a tendency to attach themselves to the proboscis of the female as

soon as they hatch. If given a chance to do so and if they stick to

the proboscis for more than three days they will develop into males,

which soon afterwards creep into the female where they continue their

parasitic existence. If, however, no adult female _Bonellia_ is put

into the aquarium in which the larvæ hatch, about ninety per cent. of

the larvæ will, after a period of rest, develop into females; the rest

develop into males. Those which develop into females will often show a

primary maleness which may manifest itself in the production of sperm

or of other secondary male sexual characters. This tendency is stronger

the longer the period of rest lasts. If the larvæ are allowed to

settle on the proboscis of the adult female but are removed too early

hermaphrodites are produced having male and female characters mixed.

Baltzer has suggested on the basis of some observations that the larvæ

while on the proboscis of the female absorb some substance secreted by

the proboscis, and this substance accelerates the further development

into a male and suppresses the female tendency. If this substance from

the proboscis does not reach the larvæ the tendency to become males is

gradually suppressed in the majority and only a few develop into pure

males or protandric hermaphrodites, while the female characters are

given a chance to develop. Baltzer assumes, therefore,--as it seems to

us correctly--that in all larvæ the tendency for both sexual characters

is present, that they are, in other words, hermaphrodites, but the

chance for the suppression of one and the development of the other

group of characters can be influenced by certain chemical substances

which the larva may take up.[190]

[190] Baltzer, F., _Mitteil. d. zoölog. Station_, Neapel, 1914, xxii.

Giard has studied the effects of a curious form of castration brought

about by parasites, which is followed by a change in the sexual

character of the castrated animal. The phenomenon is very striking

in certain forms of crabs when they are attacked by a parasitic

crustacean, _Sacculina_. The two sexes differ in the crab _Carcinus

mænas_ by the form of the abdomen, but when a male is attacked by

the parasite its abdomen assumes the female shape. Smith observed in

another crab that in such cases even the abdominal appendages of the

male may be transformed into those of a female. The transformation is

so complete that the older observers had reached the conclusion that

the parasite attacked only the females, since they overlooked the fact

that the castration by the parasite transformed the secondary sexual

characters of the male into those of a female.

Giard observed that in a diœcious plant, _Lychnis dioica_, a parasitic

fungus brings about the transformation of the host into a hermaphrodite.

G. Smith has discovered a fact which shows that chemical changes must

underlie these morphological transformations of primary or secondary

sexual characters. He noticed that in male crabs the presence of the

parasite _Sacculina_ changes the contents of the fatty constituents

in the blood, making them equal to that of the female. Vaney and

Meignon had previously shown that during the chrysalid stage the female

silkworms have always more glycogen and less fat than the males. The

castration by parasites is paralleled by what Caullery calls the

castration by senility.[191] In certain birds and also in mammals at

the time when the sexual glands cease to function certain secondary

sexual characters of the other sex make their appearance. The most

common case is that certain secondary male characters appear in the old

female (exceptionally also in the young female with abnormal ovaries)

(arrhenoidy). Thus old female pheasants assume the plumage of the

male, and in the human female after the menopause and especially among

sterile women a beard may begin to grow. The opposite phenomenon, the

old male assuming female characters, is not so common. Very interesting

observations on changes in the plumage of castrated fowl have recently

been made by Goodale.[192]

[191] Caullery, M., _Les Problèmes de la Sexualité_. Paris, 1913.

[192] Goodale, H. D., _Biol. Bull._, 1916, xxx., 286.

It had long been observed by cattle breeders that in the case of twins

of different sex the female--the so-called free-martin--is usually

sterile. F. Lillie[193] has recently discovered the cause of this

interesting phenomenon. Such twins originate from two different eggs

since the mother has two corpora lutea, one in each ovary. In normal

single pregnancies in cattle there is never more than one corpus luteum

present. The two eggs begin to develop separately in each horn of the

uterus.

[193] Lillie, F., _Science_, 1916, xliii., 611.

The rapidly elongating ova meet and fuse in the small body of the

uterus at some time between the 10 mm. and the 20 mm. stage. The

blood-vessels from each side then anastomose in the connecting part

of the chorion; a particularly wide arterial anastomosis develops,

so that either fetus can be injected from the other. The arterial

circulation of each also overlaps the venous territory of the

other, so that a constant interchange of blood takes place. If both

are males or both are females no harm results from this; but _if

one is male and the other female, the reproductive system of the

female is largely suppressed, and certain male organs even develop

in the female. This is unquestionably to be interpreted as a case

of hormone action._

The reproductive system of these sterile females is for the most

part of the female type, though greatly reduced. The gonad is the

part most affected; so much so that most authors have interpreted

it as testis.

It should be added, however, that this result cannot at present be

generalized, since in the hermaphrodites the specific hormones of both

sexes must circulate without suppressing each other’s efficiency.

All these facts indicate that certain substances secreted by the

ovaries or testes may inhibit the development of certain sexual

characters of the opposite sex. When these inhibitions are partly or

entirely removed the secondary sexual characters of the opposite sex

may appear. This fact may also be interpreted as an indication of a

latent hermaphroditism and if this be correct the real and latent

hermaphrodites differ only by the degree of inhibition for one sex,

this inhibition being lacking or less complete in the real than in the

latent hermaphrodite.

In the light of this conclusion the observations on the regeneration

of both ovaries and testicles which Janda observed in a hermaphroditic

worm, _Criodrilus lacuum_,[194] is no longer so mysterious. This

worm normally possesses in the segments near the head a pair of

ovaries and several pairs of testes. Janda found that if the anterior

parts containing the gonads of these worms are cut off a complete

regeneration takes place, including both types of gonads, ovaries

as well as testes. As a rule, more than one pair of ovaries appear

in the regenerated piece. This important experiment shows that in a

hermaphrodite both types of sex organs can be produced from body cells

or from latent buds resembling body cells. This phenomenon would be

intelligible on the assumption that in the body of a hermaphrodite

substances circulate which favour the development of both types of sex

organs, while in a diœcian animal probably only one type of sex organ

would be developed; the formation of the other being inhibited.

[194] Janda, V., _Arch. f. Entwcklngsmech._, 1912, xxxiii., 345;

xxxiv., 557.

Richard Goldschmidt has discovered in his breeding experiments on

the gipsy-moth (_Lymantria dispar_) a phenomenon which will probably

throw much light on the physiology of sex determination. He found

that certain crosses between the Japanese and the European gipsy-moth

do not give pure sexes, males or females, but mixtures of the sexual

characters of both sexes, and this mixture is a very definite one for

definite crosses. These differences are such that it is possible to

grade the hybrids according to their manifestations of maleness or

femaleness, both in morphological characters and instincts. Goldschmidt

calls this peculiar phenomenon intersexualism, and its essential

feature is that the various degrees of intersexualism can be produced

at will by the right combination of races.

Female intersexualism begins with animals which show feathered

antennæ of medium size (feathered antennæ are a male character),

but which are otherwise entirely female in appearance except

that they produce a smaller number of eggs which are fertilized

normally. In the next stage patches of the brown male pigment

appear on the white female wings in steadily increasing quantity.

The instincts are still female, the males are attracted and

copulate. But the characteristic egg sponge laid by the animal

contains nothing but anal hairs in spite of the fact that the

abdomen is filled with ripe eggs. In the next stage whole sections

of the wings show male colouration, with cuneiform female sectors

between, the abdomen becomes smaller, contains fewer ripe eggs,

the instincts are only slightly female, the males are attracted

very little, and reproduction is impossible. In the next stage the

male pigment covers practically the whole wing, the abdomen is

almost male, but still contains ovaries with a few ripe eggs, the

instincts are intermediate between male and female. Then follow

very male-like animals which still show in different organs their

female origin and have rudimentary ovaries.... The end of the

series is formed by males, which show in some minor characters,

such as the shape of wings, still some traces of their female

origin.

The series of the male intersexes starts with males showing a few

white female spots on their wings. These become larger and larger,

the amount of brown pigment correspondingly decreasing.... Hand in

hand with this the abdomen increases in size, reaching in the most

extreme cases two-thirds of the female size (without containing

eggs). The same is true for the instincts which become more and

more female.

(And also for the copulatory organs which also become more and more

female.)

As stated above, the main fact that every desired degree of

intersexualism can be produced at will by properly combining the races

for breeding, and the intersexual potencies of the different races has

been worked out by Goldschmidt.[195]

[195] Goldschmidt, R., _Proc. Nat. Acad. Sc._, 1916, ii., 53; _Ztschr.

induct. Abstammungslehre_, 1912, vii., and 1914, xi.

6. The relation between chemical substances circulating in the

body--either derivatives of food taken up from without or of chemical

compounds formed naturally inside the body--and the production of

sexual characters is best shown in the polymorphism found among the

social ants, bees, and wasps. Here we have, as a rule, in addition

to the two sexes a third one, the workers, which are in reality

rudimentary and for that reason sterile females. They differ more

or less markedly from both the typical male and female in their

external form, and, as a rule cannot copulate owing to their deficient

structure. This third sex, the sterile neuters, can be transformed

at desire into sexual females in certain species, as P. Marchal has

demonstrated. He worked with a form of social wasps in which the

workers are sterile and smaller than the real females. In such a

society of wasps all the males and workers die in the fall and only

the fertilized females survive, each one founding a new nest in the

following spring. From the first eggs laid, workers arise, small in

stature and sterile; these workers are nourished by their mother.

Then these workers take care of the feeding of all those larvæ which

arise from the eggs which their mother continues to lay. Throughout

the spring only workers arise from the eggs. The males appear in the

summer, the real females towards the end of the season when the sexes

copulate.

Marchal isolated a number of the sterile workers, providing them with

food but giving them no larvæ to raise. He found that the workers

which thus far had been sterile became fertile, producing, however,

only males. This latter fact is easily understood from what has

been said regarding the bees, namely, that the female produces only

one type of eggs, hence the unfertilized egg can give rise only to

males. The astonishing or important point is that the ovaries of the

workers begin to develop as soon as they no longer have a chance to

nourish the larvæ, provided the food which would have been given to

the larvæ is now at their disposal. In other words, the development

of their ovaries is the outcome of eating the food which under

normal conditions they would have given to the larvæ. The food must,

therefore, contain a substance which induces the development of eggs.

The natural sterility of the neuters or workers is, therefore, to use

P. Marchal’s expression, a case of “food castration,” (“castration

nutriciale”).[196] The workers originate from fertilized eggs and are

therefore females, but for the full development of the ovaries and the

other sexual characters something else besides the XX chromosomes is

needed and this is supplied in this case by the quantity or quality of

the food. May we not conclude that the same thing may happen generally,

except that these substances are formed by the body under the normal

conditions of nutrition through the influence of constituents of the

second X chromosome?

[196] This account of Marchal’s beautiful experiments is taken from

Caullery, M., _Les Problèmes de la Sexualité_. Paris, 1913.

It is known that the future queens among the bees receive also a

special type of food which the workers do not receive. Again the idea

of “food castration” of the latter is suggested.

In rotifers Whitney[197] has shown that the cycle in the production

of males and females can be regulated by the food. In some species a

scanty supply of green flagellates produced purely female offspring,

while a copious diet of the same green flagellates produced a

predominance of male grandchildren, sometimes as high as ninety-five

per cent. This was confirmed by Shull and Ladoff.[198]

[197] Whitney, D. D., _Science_, 1916, xliii., 176.

[198] Shull, A. F., and Ladoff, S., _Science_, 1916, xliii., 177.

7. The effects of the removal of the ovaries or testes upon the

development of secondary sexual characters differ for different

species. In insects the secondary sexual characters are not altered

by an operative removal of the sexual glands as in the caterpillar,

_e. g._, _Ocneria dispar_, according to Oudemans. This result has

been invariably confirmed by all subsequent workers, especially by

Meisenheimer. Crampton grafted the heads of pupæ of butterflies upon

the bodies of other specimens of the opposite sex, but the sexual

characters of the head remained unaltered.

In vertebrates, however, there exists a distinct influence of a

secretion from the sexual glands upon the development of certain of

the secondary sexual characters, which do not develop until sexual

maturity. In a way the observations on arrhenoidy and thelyidy referred

to above are indications of this influence.

Bouin and Ancel had already suggested that the sexual glands of

mammals have two independent constituents, the sexual cells and the

interstitial tissue; and that the latter tissue is responsible for the

development of the secondary sexual character. This has been proved

definitely by Steinach,[199] who showed that when young rats are

castrated certain secondary sexual characters are not fully developed.

The seminal vesicles and the prostate remain rudimentary and the

penis develops incompletely. Such animals when adult recognize the

female and seem to follow it, but do not persist in their attention and

neither erection nor cohabitation occurs. When, however, the testes

are retransplanted into the muscles of the castrated young animal

(so that they are no longer connected with their nerves) seminal

vesicles, prostate, and penis develop normally, and these animals

show normal sexual ardour and cohabitate with a female although the

female cannot become pregnant since the males cannot ejaculate any

sperm. When the retransplanted testes were examined it was found that

all the sperm cells had perished, only the interstitial tissue of the

testes remaining. It was, therefore, proved that the development of

the seminal vesicles, the prostate, the penis, and the normal sexual

instincts and activities depends upon the internal secretions from this

interstitial tissue and not upon the sex cells proper. This agrees with

the conclusions at which Bouin and Ancel had arrived by ligaturing the

vasa deferentia of male animals.

[199] Steinach, E., _Zentralbl. f. Physiol._, 1910, xxiv., 551; _Arch.

f. d. ges. Physiol._, 1912, cxliv., 72.

Steinach in another series of experiments castrated young male rats and

transplanted into them the ovaries of young females. These ovaries did

not disintegrate, the eggs remaining, and corpora lutea were formed. In

such feminized individuals the seminal vesicles, prostate, and penis

did not reach their normal development, and it was thereby proved that

the internal secretions from the ovary do not promote the growth of

the secondary sexual male characters. On the contrary, Steinach was

able to show that the growth of the penis was directly inhibited by the

ovary, since in the feminized males this organ remained smaller than in

the merely castrated animals. On the other hand the infantile uterus

and tube when transplanted into the young male with the ovaries grow

in a normal way, and Steinach thinks that pregnancy in such feminized

males is possible if sperm be injected into the uterus. In some regards

the feminized males showed the morphological habitus of females. Soon

after the transplantation of ovaries into a castrated male the nipples

of its mammary glands begin to grow to the large size which they have

in the female and by which the two sexes can easily be discriminated.

In addition the stronger longitudinal growth of the body in the male

does not occur in the feminized specimens, the body growth becomes

that of a female; and likewise the fat and hair of the feminized male

resemble that of a real female.

While the castrated males show an interest in the females, the

feminized males are absolutely indifferent to females and behave

like them when put together with normal males; and, what is more

interesting, they are treated by normal males like normal females. The

sexual instincts have, therefore, also been reversed in the feminized

males by the substitution of ovaries for testes.

The inhibition of the growth of the penis by the ovary is of

importance; it supports the idea already expressed that in

hermaphrodites this inhibition of the growth of the secondary organs of

the other sex is only feeble or does not exist at all.

We may finally ask whether there is any connection between the

cytological basis of sex determination by special sex chromosomes and

the physiological basis of sex determination by specific substances or

internal secretions. It is possible that the sex chromosomes determine

or favour, in a way as yet unknown, the formation of the specific

internal secretion discussed in the second part of this chapter. In

this way all the facts of sex determination might be harmonized, and

it may become clear that when it is possible to modify secretions by

outside conditions or to feed the body with certain as yet unknown

specific substances the influence of the sex chromosomes upon the

determination of sex may be overcome.

CHAPTER IX

MENDELIAN HEREDITY AND ITS MECHANISM[200]

_I_

1. The scientific era of the investigation of heredity begins with

Mendel’s paper on plant hybridization which was not appreciated by

his contemporaries. Mendel invented a method for the quantitative

study of heredity which consisted essentially in crossing two forms of

peas differing only in one well-defined hereditary character; and in

following statistically and separately the results of this crossing and

that of the inbreeding of the second and third generations of hybrids.

This led him to the recognition of one essential feature of heredity;

namely, that while the hybrids of the first generation are all alike,

each hybrid produces two types of sex cells in equal numbers, one for

each of the pure breeds which has been used for the crossing. This

takes place not only when the forms used for the crossing differ in

regard to one character only but also if they differ for two or more

characters. The statement made is Mendel’s law of heredity, or, more

correctly, Mendel’s law of the segregation of the hereditary characters

of the parents in the sex cells of the hybrids.[201] Mendel’s law

allows us to tabulate and calculate beforehand the relative number of

different forms which appear if the offspring of a mating of two

varieties are bred among themselves.

[200] For the literature on the subject the reader is referred to

Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B.,

_The Mechanism of Mendelian Heredity_. New York, 1915.

[201] Mendel, G., “Experiment in Plant-Hybridization,” translated in

W. Bateson’s classical book on Mendel’s _Principles of Heredity_.

Cambridge, 1909.

In order to do this it must be remembered also that while in some

cases the hybrid is an intermediate between the two parent forms, in

other cases it cannot be discriminated from one of the two parent

forms. In such cases the character which appears in the hybrid was

called by Mendel the dominant character and the one which disappeared

the recessive character. According to Bateson, who was the first to

systematize the phenomena of Mendelian heredity, recessiveness means

generally the absence of a character which is present in the dominant

type. When, _e. g._, the cross between a tall and a dwarf form of pea

gives in the first generation only tall peas, on the basis of the

presence and absence theory the dominant form contains a factor for

growth which is lacking in the dwarf form. While this theory fits

many cases it meets with difficulties in others. Thus the presence of

a factor for pigment should be dominant over the absence of such a

factor, which is usually the case, inasmuch as the cross of a coloured

rat or rabbit with an albino is black or coloured. There is, however,

also a case where whiteness is dominant over colour, as we shall see

later. This fact does not necessarily contradict the presence and

absence theory.[202]

[202] The reader will find a critical discussion of the presence and

absence theory on page 220 of Morgan, Sturtevant, Muller, and Bridges,

_The Mechanism of Mendelian Heredity_. New York, 1915.

When two pure breeds of parents differ in one character, _e. g._,

two varieties of beans, one with a violet the other with a white

flower, the cross between the two species (the F₂ generation) has pale

violet flowers, approximately intermediate between the two parents.

If these hybrids are bred among themselves the offspring is called

the F₂ generation. According to Mendel’s law the hybrids of the first

F₁ generation all have two kinds of eggs in equal numbers, one kind

representing the pure breed of the parents with violet, the other of

the pure breed with white flowers. The same is true for the pollen

cells. Hence the following possible combinations must appear in the

offspring when the pale violet hybrids are inbred:

violet white .... eggs

|\ /|

| X |

|/ \|

violet white .... pollen

The four possible combinations are: (1) violet--violet; (2)

violet--white; (3) violet--white; (4) white--white. The first will

result in pure violet flowers, the fourth in pure white, and the second

and third in pale violet flowers. Since all four combinations will

appear in equal numbers when the number of crossings is sufficiently

large the numerical result will be:

violet: pale violet: white = 1:2:1

Fifty per cent. of the F₂ generation will be pale violet, 25 per cent.

violet, and 25 per cent. white. The violets and whites each will breed

true when bred among themselves since they are pure, and produce only

one type of eggs and pollen. The pale violets are hybrids and will

again produce the two types of eggs and pollen, that is, if bred among

themselves will again give violets, pale violets, and whites in the

ratio 1:2:1. This the experiment confirms.

As has been stated, it not infrequently happens that all the hybrids

of the first generation are alike. In such cases the one character

is “recessive,” _i. e._, overshadowed or covered by the other the

“dominant” character, which alone appears in the hybrids. Thus when

Mendel crossed peas having round seeds with peas having angular seeds

all the hybrids had round seeds. The round form is dominant, the

angular recessive, _i. e._, all the hybrids have round seeds. When

these hybrids were bred among themselves the next generation produced

round and angular seeds in the ratio of 3:1 (5474 round to 1850

angular). The explanation is as follows. Let R denote round, A angular

character; the pure breeds of parents have the gametic constitution

RR and AA respectively. When crossed, all the offsprings have the

constitution RA and since A is recessive this hybrid generation

resembles the pure RR parents. The F₁ generation produces two kinds

of eggs R and A and two kinds of pollen R and A in equal numbers, and

these if inbred give the following four combinations in equal numbers:

RR, RA, AR, AA.

Since RA, AR, and RR all give round seeds the F₂ generation produces

round seeds to angular seeds in the ratio of 3:1. The two organisms

with the gametic constitution RR and RA look alike, yet they are

different in regard to heredity. The gametically pure form RR is called

homozygous, the impure form RA heterozygous.

2. W. S. Sutton[203] was the first to show that the behaviour of

the chromosomes furnishes an adequate basis on which to account for

Mendel’s law of the segregation of the characters in the sex cells

of the hybrids. If we disregard the cases of parthenogenesis and the

X chromosomes, we may state that each species is characterized by a

definite number of chromosomes, _e. g._[204]

[203] Sutton, W. S., “The Chromosomes in Heredity,” _Biol. Bull._,

1904, iv., 231.

[204] Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C.

B., _Mechanism of Mendelian Heredity_. New York, 1915, p. 26.

man (probably) 24 corn 20

mouse 20 evening primrose 7

snail (_Helix hortensis_) 22 nightshade 36

potato beetle 18 tobacco 24

cotton 28 tomato 12

four o’clock 16 wheat 8

garden pea 7

In the fertilization of the egg the number of chromosomes is doubled

(if we disregard for the moment the complication caused by the X and

Y chromosomes which was considered in the previous chapter). It was

noticed by Montgomery that each chromosome had a definite size and

individuality, and he suggested that homologous chromosomes existed in

sperm and egg and that in fertilization the homologous chromosomes of

egg and sperm always joined and fused in the special stage designated

as synapsis, which will interest us later. On the basis of this

suggestion Sutton developed the chromosome theory of the mechanism of

Mendelian heredity or segregation.

According to this theory, all the cells of an individual (inclusive of

the egg cells and sperm cells) have two sets of homologous chromosomes,

one from the father, the other from the mother. Before the egg and

sperm are ready for the production of a new individual, each loses

one set of homologous chromosomes in the so-called reduction division,

but the lost set is made up indiscriminately of maternal as well as

paternal chromosomes, so that while one egg retains the maternal

chromosome _A_ the other will retain the paternal one, and so on.

If before the reduction division all the eggs had the chromosome

constitution _AA₁_, _BB₁_, _CC₁_, _DD₁_ (where _A_ _B_ _C_ _D_ are

the paternal and _A₁_ _B₁_ _C₁_ _D₁_ the maternal chromosomes), after

the reduction division each daughter cell has a full set of four

chromosomes, but maternal and paternal mixed. Thus the one cell may

have _AB₁__CD₁_, the other _A₁__B₁__C₁__D₁_, etc. This, according to

Sutton, is the basis of the Mendelian heredity. Suppose the determiner

of a certain character (violet colour of flower in the bean) is located

in a chromosome _A_ of this species. The homologous chromosome in beans

with white colour may be designated as _a_. According to the chromosome

theory of Mendelian heredity _a_ differs from _A_ in one point, though

this difference is probably only of a chemical character and not

visible.

If an egg with _A_ is fertilized by a pollen with _a_ (or _vice

versa_), after fertilization the chromosome constitution of the

fertilized egg is _Aa_. All the other homologous chromosomes are

identical and therefore need not be considered. All the nuclei of the

F₁ generation have the chromosome constitution _Aa_. All will form

eggs and pollen with nuclei of the same chromosome constitution _Aa_,

but all these sex cells will go through the maturation division before

they are fertilized; and this reduction division leads to the existence

of two kinds of eggs in equal numbers, one containing only the _A_, the

other only the _a_ chromosome; and the same happens in the pollen. When

therefore the hybrids F₁ are mated among themselves, the following four

chromosome combinations will be produced:

Eggs Pollen

A a A a

[Transcriber's note: Diagram illustrating the four possible pairings

of chromosomal characters]

Possible combinations in fertilized eggs

_AA_, _Aa_, _aa_, in the ratio 1:2:1.

Now this is exactly the ratio of Mendelian heredity in the F₂

generation. The plant with the chromosome constitution _AA_ will form

violet flowers, those with the chromosome constitution _Aa_ will form

pale violet flowers, and those with the chromosome constitution _aa_

will form white flowers.

To quote Sutton’s words:

The result would be expressed by the formula _AA:_ _Aa:_ _aa_

which is the same as that given for any character in a Mendelian

case. Thus the phenomena of germ cell division and of heredity are

seen to have the same essential features viz., purity of units

(chromosomes, characters) and the independent transmission of the

same; while as a corollary it follows in each case that each of the

two antagonistic units (chromosomes, characters) is contained by

exactly half the gametes produced.

It is obvious that Sutton by this idea did for heredity in general what

McClung had done for sex determination or sex heredity, that is, he

showed that the numerical results obtained in Mendelian heredity can

be accounted for on the basis that factors for hereditary characters

are carried by definite chromosomes. The cytological basis of sex

determination becomes only a special case of the cytological basis of

Mendelian heredity. In the examples quoted the plants giving rise to

violet and to white flowers are homozygous for the colour of flower

having the chromosome constitution _AA_ and _aa_ respectively; while

the plants with pale violet flowers are heterozygous, having the

chromosome constitution _Aa_ in their nuclei. The former give rise to

identical sex cells _A_ and _A_ or _a_ and _a_; while the heterozygous

plants give rise to different sex cells _A_ and _a_.

From this point of view in _Drosophila_ (and very probably also in

man) the female is homozygous for sex having in all its cells the

critical chromosome constitution XX and giving rise to one type

of eggs only, each with one X chromosome; while the male in these

forms is heterozygous for sex having in all its cells the chromosome

constitution XY and forming two different types of spermatozoa in

equal numbers X and Y. In _Abraxas_ and in the fowl the female is

heterozygous for sex and the male homozygous.

3. If the chromosomes are the vehicle for Mendelian heredity it should

be possible to show that the various hereditary characters which follow

Mendel’s law must be distributed over the various chromosomes; and it

should be possible to find out which characters are contained in the

same chromosome. It has already been stated that sex-linked heredity

is intelligible on the assumption that the X chromosome carries the

sex-linked characters. T. H. Morgan and his pupils have shown with the

greatest degree of probability that corresponding linkages occur in the

other chromosomes and that there are in _Drosophila_ exactly as many

groups of linkage as there are different chromosomes, namely four.[205]

[205] Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C.

B., _The Mechanism of Mendelian Heredity_. New York, 1915.

Mendel had found that when he crossed two species of peas differing in

regard to two pairs of characters, he obtained in the F₂ generation

results which he calculated on the assumption that the segregation

of the two pairs of characters in the sex cells of the hybrids took

place independently of each other. To illustrate by an example: When

crossing a yellow round pea with a green wrinkled variety in which the

characters round and yellow are dominant, green and wrinkled recessive,

all the hybrids of the F₁ generation had the characters round and

yellow. When these were inbred the F₂ generation produced four types of

seed in the ratio 9: 3: 3: 1, namely:

(1) yellow round (315 seeds)

(2) yellow wrinkled (101 seeds)

(3) green round (108 seeds)

(4) green wrinkled (32 seeds)

The explanation according to Mendel’s theory is as follows: Since the

segregation of each pair of characters occurs independently, there

must be 3 yellow to 1 green and also 3 round to 1 wrinkled in the F₂

generation. The yellow will, therefore, be round and wrinkled in the

ratio of 3:1, which will give 9 yellow round to 3 yellow wrinkled. The

green will also be round and wrinkled in the ratio of 3:1, which will

give 3 green round to 1 green wrinkled, which is the ratio of 9: 3: 3:

1 found by Mendel.

On the basis of the chromosome theory the following explanation could

be given of this numerical relation. The peas with yellow round

seeds have sex cells with a factor for both yellow and for round

in two different chromosomes; these two different chromosomes we

will designate with Y and R. The peas with green and wrinkled seeds

will have in their sex cells factors for these characters in two

homologous chromosomes g and w, where g is the homologue of Y and w

of R. The cells of the hybrids of the F₁ generation will have the

chromosome constitution Yg Rw, where Y and g and R and w are homologous

chromosomes which will lie alongside each other YRgw[Transcriber's

note: displayed in the text as YR over gw, i.e. like a fraction but

without a dividing bar]. In the formation of sex cells a reduction of

these four chromosomes to two takes place whereby, according to the

theory of Sutton, the following two types of separation can take place:

YR and gw, or gR and Yw. (A separation into Yg and Rw is impossible

since the division takes place only between homologous chromosomes.)

Hence there will be four types of eggs, YR, gw, gR, and Yw and the same

four types of pollen cells. The F₂ generation will produce the sixteen

possible combinations in equal numbers: namely,

YRYR YRgw YRgR YRYw

gwYR gwgw gwgR gwYw

gRYR gRgw gRgR gRYw

YwYR Ywgw YwgR YwYw

Since w and g are recessives and therefore disappear when in

combination with their respective dominants Y and R the result will

be 9 YR (yellow round), 3 Yw (yellow wrinkled), 3 Rg (round green),

and 1 gw (green wrinkled) as Mendel actually observed and as all

investigators since have confirmed.

Bateson made the discovery that these Mendelian ratios 9: 3: 3: 1 did

not always occur when forms differing in two characters were crossed.

He found typical and very constant deviations from this ratio in

definite cases and these cases he interpreted as being due to “gametic

coupling.”

These phenomena demonstrate the existence of a complex

interrelation between the factorial units. This interrelation is

such that certain combinations between factors may be more frequent

than others. The circumstances in which this interrelation is

developed and takes effect we cannot as yet distinguish, still less

can we offer with confidence any positive conception as to the mode

in which it is exerted.[206]

[206] Bateson, W., _loc. cit._, p. 157.

Morgan has given an ingenious explanation of these deviations on the

basis of the chromosome theory of Mendelian heredity. He assumes

that they occur in those cases where the two or more characters are

contained in the same chromosome. In that case the two factors lying

in the same chromosome should generally be found together. Such was

the case for instance in the experiments with flies having red eyes

and yellow body colour _versus_ white eyes and grey body colour,

the character for white eyes and yellow body being located in the X

chromosome (see preceding chapter), or in the experiments on _Abraxas_.

These phenomena are called linkage, and the numerical results of

linkage were given in the preceding chapter in connection with the

crossing of sex-linked characters.

We have already mentioned that before the maturation division occurs

the homologous maternal and paternal chromosomes fuse--the so-called

synapsis of the cytologists--and afterward separate again. It had

been observed by Janssens that in this stage of fusion and subsequent

separation a partial twisting and a partial exchange between two

chromosomes may take place. Morgan assumes that this exchange accounts

for certain deviations in the ratio of linkage. If in Fig. 40 the white

and black signify two homologous chromosomes I and I₁ containing the

two pairs of homologous factors AB and ab respectively, the synapsis

state would be as in Fig. 41. If the separation were complete, either

I or its homologue I₁ might be lost in the maturation division of

the egg. If, however, the synapsis is slightly irregular, as in Fig.

42, where the chromosomes are slightly twisted, I and I₁ will not

separate completely but an exchange will take place, part of I₁ and

I becoming exchanged. This would result in the formation of two mixed

chromosomes Ab and aB (Fig. 42). This partial exchange of homologous

chromosomes, which Morgan calls “crossing over,” occurs, as he found in

_Drosophila_, in the egg only, not in the maturation division of the

sperm. He informs me that in the silkworm moth Tanaka found that it

occurs only in the male, while in _Primula_ it takes place both in the

ovules and in the pollen as shown by Gregory.

[Illustration: FIG. 40]

[Illustration: FIG. 41]

[Illustration: FIG. 42]

Morgan and his fellow-workers have put this theory to numerous tests by

breeding experiments and the results have fully supported it. According

to the chromosome theory linkage should occur only when factors lie in

the same chromosome. Hence it should be possible, on the basis of this

linkage theory, to foretell how many linkage groups there may occur in

a species; namely, as many as there are chromosomes. In _Drosophila_

there are four pairs of chromosomes, and Morgan and his fellow-workers

found only four groups of linked characters.[207] This agreement can be

no mere accident.

[207] The number of hereditary characters examined to test the theory

was over 130.

Carrying the assumption still farther, these authors were able to

show that each individual character has in all probability a definite

location in the chromosome, so that it seems as if each individual

chromosome consisted of a series of smaller chromosomes, each of which

may be a factor in the determination of a hereditary character which is

transmitted according to Mendel’s law of segregation. Biology has thus

reached in the chromosome theory of Mendelian heredity an atomistic

conception, according to which independent material determiners for

hereditary characters exist in a linear arrangement in the chromosomes.

_II_

4. We are not concerned in this volume with the many applications of

the theory of heredity to the breeding of plants, animals, and man;

the reader will find a discussion of these topics in the numerous

writings of the special workers on genetics.[208] We are, however,

interested in the bearing this work has on the conception of the

organism. Two questions present themselves: Is the organism nothing but

a mosaic of hereditary characters determined essentially by definite

elements located in the chromosomes; and if this be true, what makes

a harmonious whole organism out of this kaleidoscopic assortment?

We call it a kaleidoscopic assortment since a glance at the list of

hereditary characters found in one chromosome, according to Morgan,

shows that there is apparently no physiological or chemical connection

between them, and second: How can a factor contained in the chromosome

determine a hereditary character of the organism? To the first question

we venture to offer the answer which has been already suggested in

various chapters of this book, that the cytoplasm of the egg is the

future embryo in the rough; and that the factors of heredity in the

sperm only act by impressing the details upon the rough block. This

metaphor will receive a more definite meaning by the answer to the

second question. The characters which follow Mendelian heredity are

morphological features as well as instincts. For the former we have

already had occasion to show in previous chapters to what extent they

depend upon the internal secretions or the existence of specific

compounds in the circulation, and the same is true for the instincts

(Chapters VIII and X). This then leads us to the suggestion that

these determiners contained in the chromosomes give rise each to the

formation of one or more specific substances which influence various

parts of the body. We probably do not notice all the effects in each

case, but when a special organ is affected in a conspicuous way, we

connect the factor with this organ or the special feature of the organ

which is altered, and speak of a determiner or factor for that organ,

or for one of its characters. We also understand in this way why

outside conditions should be able to overcome the hereditary tendency

in certain cases, for instance why the influence of certain hereditary

factors for pigmentation should depend upon temperature as E. Baur

observed.

[208] Bateson, W., _Mendel’s Principles of Heredity_, 3d ed., 1913;

Davenport, Chas. B., _Heredity in Relation to Eugenics_, 1911. Pearl,

R., _Modes of Research in Genetics_.

The view, according to which the determiners in the chromosomes only

tend to give special characters to the embryo or to the adult while

the cytoplasm of the egg may be considered the real embryo, receives

some support from the fact that the first development of the egg is

purely maternal, even if the egg nucleus has been replaced by sperm

of a different species. If an egg of a sea urchin be cut into two

pieces, one with and one without a nucleus, and the enucleated piece

be fertilized with the sperm of a different species of sea urchin, the

blastula and gastrula stages are purely maternal and only the skeleton

of the pluteus stage begins to betray the influence of the foreign

sperm inasmuch as this skeleton is purely paternal, according to

Boveri. In all experiments on hybridization it has been found that the

rate of cell division of the egg is a purely maternal character. Thus

when fish eggs of a species, in which the rate of first segmentation of

the egg is about eight hours, are fertilized with sperm of a species

for which the same process requires about thirty minutes or less at

the same temperature, the rate of segmentation is again about eight

hours. There is then no chromosome influence noticeable in the early

development.

When two forms of sea urchins, _Strongylocentrotus_ _franciscanus_ and

_purpuratus_,[209] are crossed, certain features of the skeleton of the

embryo, _e. g._, the so-called cross-bars, are a dominant, inasmuch as

they are found in _purpuratus_ and both the crosses, while they are

absent in _franciscanus_. The development prior to the formation of the

skeleton is purely maternal. These observations again lend support to

the idea that the Mendelian factors of heredity must have the embryo

to work on and that the organism is not to be considered a mere mosaic

of Mendelian factors. This is further supported by the idea that the

species specificity resides in the proteins of the unfertilized egg

(see Chapter III), and it is quite likely that this species specificity

decides which type of animal should arise from an egg.

[209] Loeb, J., King, W. O. R., and Moore, A. R., _Arch. f.

Entwcklngsmech._, 1910, xxix., 354. These experiments have been

repeated at different seasons of the year and in different years and

have been found to be constant.

The idea had been suggested that the factors which determine the future

character might be ferments or enzymes, or substances from which such

ferments develop. A. R. Moore[210] pointed out that the cross-bars in

the skeleton of the hybrid between _S. purpuratus_ and _franciscanus_

develop more slowly than in the pure breed and that this should be

expected if the determiners were enzymes. Since the pure _purpuratus_

has two determiners for the development of the cross-bars (from both

egg and sperm), the hybrids only one (from either egg or sperm), the

pure _purpuratus_ should have twice the enzyme mass of the hybrid.

It is known that the velocity of a chemical reaction increases in

proportion with the mass (or in some cases in proportion with the

square root of the mass) of the enzyme; the cross-bars should therefore

develop faster in the pure than in the hybrid breeds, as was observed

by Moore. It was, however, not possible to obtain quantitative data.

[210] Moore, A. R., _Arch. f. Entwcklngsmech._, 1912, xxxiv., 168.

On the other hand, it is obvious that this reasoning would not hold

for all cases. Thus when beans with violet flowers are crossed with

white-flowered beans the hybrids are pale blue, which indicates that

the hybrids have less pigment than the pure violet. Now we know that

the mass of enzyme does not influence the chemical equilibrium but only

the velocity of the reaction. The hybrids and pure violets differ,

however, in the mass of violet pigment formed, that is to say, in

regard to the equilibrium. Hence the idea that the determiners are

enzymes or give rise to enzymes is probably not applicable to cases of

this type.

The experiments on the heredity of pigments are at present almost the

only ones which can be used for an analysis of the chemical nature

of the character and its possible determiner. The important work

of G. Bertrand[211] and of Chodat[212] on the production of black

pigment in the cells of animals and plants with the aid of enzymes

has paved the way for such work. Bertrand has shown that tyrosine

(_p_-oxyphenylaminopropionic acid) is transformed into a black pigment

by an enzyme tyrosinase which occurs in numerous organisms and is

obviously the cause of pigment and colouration in a great number of

species. This discovery was utilized in the study of the heredity

of pigments by Miss Durham, Gortner,[213] and very recently by

Onslow.[214] The latter showed that from the skins of certain coloured

rabbits and mice a peroxidase can be extracted which behaves like a

tyrosinase toward tyrosine in the presence of hydrogen peroxide. This

peroxidase was found in the skins of black agouti, chocolate and blue

rabbits, but not in yellow or orange rabbits. The recessive whiteness

in rabbits and mice according to this author is due to the lack of the

peroxydase. There exists a dominant whiteness in the English rabbit

which is due to a tyrosinase inhibitor which destroys the activity of

the tyrosinase “and the dominant white bellies of yellow and agouti

rabbits are due to the same cause.” “Variations in coat colour are

probably due to a quantitative rather than to a qualitative difference

in the pigment present.”

[211] Bertrand, G., _Ann. d. l’Inst. Pasteur_, 1908, xxii., 381; _Bull.

Soc. Chim._, 1896, xv., 791.

[212] Chodat, R., _Arch. d. Sc. phys. et nat._, 1915, xxxix., 327.

[213] Gortner, R. A., _Trans. Chem. Soc._, 1910, xcvii., 110.

[214] Onslow, H., _Proc. Roy. Soc._, 1915, B. lxxxix., 36.

One point might still be mentioned since it may help to overcome a

difficulty in visualizing the connection between the localization

of a factor in the chromosome and the production of a comparatively

large quantity of a specific chemical compound, _e. g._, a chromogen

or a tyrosinase. We must remember that all the cells of an organism

have identical chromosomes, so that a factor for an enzyme like

tyrosinase is contained in every cell throughout the whole body.

It is likely, however, that the same factor (which we may conceive

to be a definite chemical compound) will find a different chemical

substrate to work on in the cells of different organs of the body,

since the different organs differ in their chemical composition.

Thus it is conceivable that in the production of tyrosinase or of

tyrosine not a single chromomere of one single cell is engaged, but

the sum total of all these individual chromomeres of all the cells in

one or several organs of the body. The writer has added this remark

especially in consideration of the fact that some authors seem to feel

that the chromosome conception of heredity is incompatible with a

physicochemical view of this process.

Since we have mentioned this difficulty which some writers seem to

find in the chromosome theory of Mendelian heredity, it may be added

that a single factor may suffice to determine a series of complicated

reflexes. Thus the heliotropic reactions of animals are due to the

presence of photosensitive substances, and it suffices for the

hereditary transmission of the complicated purposeful reactions based

on these tropisms that a factor for the formation of the photosensitive

substance should exist.[215]

[215] Loeb, J., “Egg Structure and the Heredity of Instincts,” _The

Monist_, 1897, vii., 481.

5. Another point should be emphasized, namely that for Mendelian

heredity it is immaterial whether the character is introduced by the

spermatozoön or by the egg. This fact which Mendel himself already

recognized is in full harmony with the conclusion that the chromosomes

and not the cytoplasm are the bearers of Mendelian heredity, since only

in respect to the chromosome constitution are egg and sperm alike,

while they differ enormously in regard to the mass of protoplasm they

carry. We can, therefore, be tolerably sure that wherever we deal with

a hereditary factor which is determined by the egg alone the cytoplasm

of the latter is partly or exclusively responsible for the result.

We have already mentioned the fact that the rate of segmentation of

the egg is such a character. Yet this character is as definite as

any Mendelian character, and it would be as easy to discriminate two

species of eggs by the time required from insemination to the beginning

of cell division as it would be by any Mendelian character of their

parents.

The application of our modern knowledge of heredity to human affairs

has been discussed in a very original way by Bateson in his address

before the British Association in Sydney to which the reader may be

referred.[216]

[216] Bateson, W., _Nature_, 1916, xciii., 674.

CHAPTER X

ANIMAL INSTINCTS AND TROPISMS[217]

1. The idea that the organism as a whole cannot be explained from

a physicochemical viewpoint rests most strongly on the existence

of animal instincts and will. Many of the instinctive actions are

“purposeful,” _i. e._, assisting to preserve the individual and the

race. This again suggests “design” and a designing “force,” which we do

not find in the realm of physics. We must remember, however, that there

was a time when the same “purposefulness” was believed to exist in the

cosmos where everything seemed to turn literally and metaphorically

around the earth, the abode of man. In the latter case, the

anthropo- or geocentric view came to an end when it was shown that the

motions of the planets were regulated by Newton’s law and that there was

no room left for the activities of a guiding power. Likewise, in the

realm of instincts when it can be shown that these instincts may be

reduced to elementary physicochemical laws the assumption of design

becomes superfluous.

[217] Ideas similar to those expressed in this chapter may be found

in the writer’s former book _Comparative Physiology of the Brain and

Comparative Psychology_, New York, 1900, and in the books by George

Bohn, _La Naissance de l’Intelligence_, Paris, 1909, and _La nouvelle

Psychologie animale_, Paris, 1911.

If we look at the animal instincts purely as observers we might well

get the impression that they cannot be explained in mechanistic terms.

We need only consider what mysticism apparently surrounds all those

instincts by which the two sexes are brought together and by which the

entrance of the spermatozoön into the egg is secured; or the remarkable

instincts which result in providing food and shelter for the young

generation.

We have already had occasion to record some cases of instincts which

suggest the possibility of physicochemical explanation; for example the

curious experiment of Steinach on the reversal of the sexual instincts

of the male whose testes had been exchanged for ovaries. There is

little doubt that in this case the sexual activities of each sex are

determined by specific substances formed in the interstitial tissue of

the ovary and testes. The chemical isolation of the active substances

and an investigation of their action upon the various parts of the body

would seem to promise further progress along this line.

Marchal’s observations on the laying of eggs by the naturally sterile

worker wasps are a similar case. The fact that such workers lay

eggs when the queen is removed or when they are taken away from the

larvæ may be considered as a manifestation of one of those wonderful

instincts which form the delight of readers of Maeterlinck’s romances

from insect life. Imagine the social foresight of the sterile workers

who when the occasion demands it “raise” eggs to preserve the stock

from extinction! And yet what really happens is that these workers,

when there are no larvæ, can consume the food which would otherwise

have been devoured by the larvæ; and some substance contained in

this food induces the development of eggs in the otherwise dormant

ovaries. What appeared at first sight as a mysterious social instinct

is revealed as an effect comparable to that of thyroid substance upon

the growth of the legs of tadpoles in Gudernatsch’s experiment (Chapter

VII).

2. If we wish to show in an unmistakable way the mechanistic character

of instincts we must be able to reduce them to laws which are also

valid in physics. That instinct, or rather that group of instincts,

for which this has been accomplished are the reactions of organisms

to light. The reader is familiar with the tendency of many insects to

fly into the flame. It can be shown that many species of animals, from

the lowest forms up to the fishes, are at certain stages--very often

the larval stage--of their existence, slaves of the light. When such

animals, _e. g._, the larvæ of the barnacle or certain winged plant

lice or the caterpillars of certain butterflies, are put into a trough

or test-tube illuminated from one side only, they will rush to the

side from which the light comes and will continue to do this whenever

the orientation of the trough or test-tube to the light is changed;

while they will be held at the window side of the vessel if the light

or the position of the vessel remains unchanged. This instinct to get

to the source of light is so strong that, _e. g._, the caterpillars of

_Porthesia chrysorrhœa_ die of starvation on the window side of the

vessel, with plenty of food close behind. This powerful “instinct”

is, as we intend to show, in the last analysis, the expression of

the Bunsen-Roscoe law of photochemical reactions. A large number

of chemical reactions are induced or accelerated by light, and the

Bunsen-Roscoe law shows that the chemical effect is in these cases,

within certain limits, equal to the product of the intensity into the

duration of illumination.

The “attraction” or “repulsion” of animals by the light had been

explained by the biologists in an anthropomorphic way by ascribing to

the animals a “fondness” for light or for darkness. Thus Graber, who

had made the most extensive experiments, gave as a result the statement

that animals which are fond of light are also fond of blue while they

hate the red, and those which are fond of the “dark” are fond of red

and hate the blue.[218] In 1888 the writer published a paper in which

he pointed out that the so-called fondness of animals for light and

blue and for dark and red was simply a case of an automatic orientation

of animals by the light comparable to the turning of the tips of a

plant towards the window of the room in which the plant is raised.[219]

[218] Graber, V., _Grundlinien zur Erforschung des Helligkeits- und

Farbensinnes der Tiere_. Prag, 1884.

[219] Loeb, J., _Sitzungsber. d. physik.-med. Gesellsch_. Würzburg,

1888. _Der Heliotropismus der Tiere und seine Übereinstimmung mit

dem Heliotropismus der Pflanzen._ Würzburg, 1889. _Arch. f. d. ges.

Physiol._, 1897, lxvi., 439.

The phenomenon of a plant bending or growing to the source of light is

called positive heliotropism (while we speak of negative heliotropism

in all cases in which the plant turns away from the light, as is

observed in many roots). The writer pointed out that animals which go

to the light are positively heliotropic (or phototropic) and do so

because they are compelled automatically by the light to move in this

direction, while he called those animals which move away from the light

negatively heliotropic; they are automatically compelled by the light

to move away from it. What the light does is to direct the motions of

the animals and to explain this the following theory was proposed.

Animals possess photosensitive elements on the surface of their bodies,

in the eyes, or occasionally also in epithelial cells of their skin.

These photosensitive elements are arranged symmetrically in the body

and through nerves are connected with symmetrical groups of muscles.

The light causes chemical changes in the eyes (or the photosensitive

elements of the skin). The mass of photochemical reaction products

formed in the retina (or its homologues) influences the central

nervous system and through this the tension or energy production of

the muscles. If the rate of photochemical reaction is equal in both

eyes this effect on the symmetrical muscles is equal, and the muscles

of both sides of the body work with equal energy; as a consequence

the animal will not be deviated from the direction in which it was

moving. This happens when the axis or plane of symmetry of the animal

goes through the source of light, provided only one source of light

be present. If, however, the light falls sidewise upon the animal,

the rate of photochemical reaction will be unequal in both eyes and

the rate at which the symmetrical muscles of both sides of the body

work will no longer be equal; as a consequence the direction in which

the animal moves will change. This change will take place in one of

two ways, according as the animal is either positively or negatively

heliotropic; in the positively heliotropic animal the resulting motion

will be toward, in the negatively heliotropic from, the light. Where

we have no central nervous system, as in plants or lower animals,

the tension of the contractile or turgid organs is influenced in a

different way, which we need not discuss here.

The reader will perceive that according to the writer’s theory

two agencies are to be considered in these reactions: first, the

symmetrical arrangement of the photosensitive and the contractile

organs, and second, the relative masses of the photochemical reaction

products produced in both retinæ or photosensitive organs at the same

time. If a positively heliotropic animal is struck by light from one

side, the effect on tension or energy production of muscles connected

with this eye will be such that an automatic turning of the head and

the whole animal towards the source of light takes place; as soon as

both eyes are illuminated equally the photochemical reaction velocity

will be the same in both eyes, the symmetrical muscles of the body will

work equally, and the animal will continue to move in this direction.

In the case of the negatively heliotropic animal the picture is the

same except that if only one eye is illuminated the muscles connected

with this eye will work less energetically. The theory can be nicely

tested for negatively heliotropic animals in the larvæ of the blowfly

when they are fully grown, and for positively heliotropic animals on

the larvæ of _Balanus_, and many other organisms.

One of the difficulties in identifying the motions of animals to or

from the light with the positive and negative heliotropism of plants

consisted in the fact that plants are mostly sessile (and respond to

a one-sided illumination with heliotropic curvatures to or from the

light), while most animals are free moving and respond to the one-sided

illumination by being turned and compelled to move to or from the

light. This difficulty was overcome by the observation that sessile

animals like the hydroid _Eudendrium_ (Fig. 43) or the tube worm

_Spirographis_ (Fig. 44) react to a one-sided illumination also with

heliotropic curvatures like sessile plants.[220] On the other hand,

it had been found before by Strassburger that free-swimming plant

organisms like the swarmspores of algæ move to or from the source of

light as do free-swimming animals.

[220] Loeb, J., _Arch. f. d. ges. Physiol._, 1890, xlvii., 391; 1896,

lxiii., 273.

[Illustration: FIG. 43]

[Illustration: FIG. 44]

3. The writer suggested in 1897[221] that the light acts chemically in

the heliotropic reactions and in 1912 that the heliotropic reactions

probably follow the law of Bunsen and Roscoe,[222] and it was possible

to confirm this idea by direct experiments.[223] This law states

that the photochemical effect of light equals _i t_ where _i_ is

the intensity of the light and _t_ the duration of illumination.

The experiments were carried out on young regenerating polyps of

_Eudendrium_ by measuring the time required to cause fifty per cent. of

the polyps to bend to the source of light. The intensity of light was

varied by altering the distance of the source of light from the polyps.

Table VI gives the result.

[221] Loeb, J., _Arch. f. d. ges. Physiol._, 1897, lxvi., 439.

[222] Loeb, J., _The Mechanistic Conception of Life_, Chicago, 1912, p.

27.

[223] Loeb, J., and Ewald, W. F., _Zentralbl. f. Physiol._, 1914,

xxvii., 1165.

TABLE VI

------------+----------------------------------------------

_Distance |_Time Required to Cause Fifty Per Cent. of the

between | Polyps to Bend towards the Source of Light_

Polyps and +----------------------+-----------------------

Source of | Observed | Calculated from

Light_ | | Bunsen-Roscoe Law

------------+----------------------+-----------------------

_Metres_ | _Minutes_ | _Minutes_

------------+----------------------+-----------------------

0.25 | 10 |

0.50 | between 35 and 40 | 40

1.00 | 150 | 160

1.50 | between 360 and 420 | 360

------------+----------------------+-----------------------

We must therefore conclude that the heliotropic curvature of the

polyps is determined by a photochemical action of the light. The

light brings about or accelerates a chemical reaction which follows

the Bunsen-Roscoe law. As soon as the product of this reaction on one

side of the polyp exceeds that on the other by a certain quantity,

the bending occurs. When the product _it_ is the same for symmetrical

spots of the organism no bending can result. This is what our theory

suggested.

It is very difficult to prove directly the applicability of the

Bunsen-Roscoe law for free-moving animals, but it can be shown that

intermittent light is as effective as constant light of the same

intensity, provided that the total duration of the illumination by the

intermittent light is equal to that of the constant light, and the

duration of the intermission is sufficiently small (Talbot’s law).

Talbot’s law is in reality only a modification of the Bunsen-Roscoe

law. Ewald has proved in a very elegant way the applicability of

Talbot’s law to the orientation of the eyestalk of _Daphnia_.[224] This

makes it probable that the law of Bunsen-Roscoe underlies generally the

heliotropic reaction of animals.

[224] Ewald, W. F., _Science_, 1913, xxxviii., 236.

It is of importance for the theory of the identity of the heliotropism

of animals and plants that in the latter organisms the law of Bunsen

and Roscoe is also applicable. This had been shown previously

by Fröschel[225] and by Blaauw.[226] In the following table are

given the results of Blaauw’s experiments on the applicability

of the Bunsen-Roscoe law for the heliotropic curvature of the

seedlings of oats (_Avena sativa_). The time required to cause

heliotropic curvatures for intensities of light varying from 0.00017

to 26520 metre-candles was measured. The product _i t_, namely

metre-candles-seconds, varies very little (between 16 and 26).

[225] Fröschel, P., _Sitzungsber. d. k. Akad. d. Wissensch._, Wien,

1908, cxvii.

[226] Blaauw, H. A., _Rec. d. travaux botaniques Neérlandais_, 1909,

v., 209.

TABLE VII

---------------+--------------+----------

I | II | III

_Duration of | _Metre- | _Metre-

Illumination_ | Candles_ | Candles-

| | Seconds_

---------------+--------------+----------

43 hours | 0.00017 | 26.3

13 " | 0.000439 | 20.6

10 " | 0.000609 | 21.9

6 " | 0.000855 | 18.6

3 " | 0.001769 | 19.1

100 minutes | 0.002706 | 16.2

60 " | 0.004773 | 17.2

30 " | 0.01018 | 18.3

20 " | 0.01640 | 19.7

15 " | 0.0249 | 22.4

8 " | 0.0498 | 23.9

4 " | 0.0898 | 21.6

40 seconds | 0.6156 | 24.8

25 seconds | 1.0998 | 27.5

8 " | 3.02813 | 24.2

4 " | 5.456 | 21.8

2 " | 8.453 | 16.9

1 " | 18.94 | 18.9

2/5 " | 45.05 | 18.0

2/25 " | 308.7 | 24.7

1/25 " | 511.4 | 20.5

1/55 " | 1255 | 22.8

1/100 " | 1902 | 19.0

1/400 " | 7905 | 19.8

1/800 " | 13094 | 16.4

1/1000 " | 26520 | 26.5

---------------+--------------+----------

It is, therefore, obvious that the blind instinct which forces animals

to go to the light, _e. g._, in the case of the moth, is identical with

the instinct which makes a plant bend to the light and is a special

case of the same law of Bunsen and Roscoe which also explains the

photochemical effects in inanimate nature; or in other words, the will

or tendency of an animal to move towards the light can be expressed in

terms of the Bunsen-Roscoe law of photochemical reactions.

The writer had shown in his early publications on light effects that

aside from the heliotropic reaction of animals, which as we now know

depends upon the product of the intensity and duration of illumination,

there is a second reaction which depends upon the sudden changes in

the intensity of illumination. These latter therefore obey a law of

the form: Effect = f (di/dt).[227] Jennings has maintained that the

heliotropic reactions of unicellular organisms are all of this kind,

but investigations by Torrey and by Bancroft[228] on _Euglena_ have

shown that Jennings’s statements were based on incomplete observations.

[227] Loeb, J., _Arch. f. d. ges. Physiol._, 1893, liv., 81; _Jour.

Exper. Zoöl._, 1907, iv., 151.

[228] Bancroft, F. W., _Jour. Exper. Zoöl._, 1913, xv., 383.

4. In these experiments only one source of light was applied. “When two

sources of light of equal intensity and distance act simultaneously

upon a heliotropic animal, the latter puts its median plane at right

angles to the line connecting the two sources of light.”[229] This

fact has been amply verified by Bohn, by Parker and his pupils, and

especially by Bradley Patten, who used it to compare the relative

efficiency of two different lights.

[229] Loeb, J., _Studies in General Physiology_, Chicago, 1905, p. 2.

The behaviour of the animals under the influence of two lights is a

confirmation of our theory of heliotropism inasmuch as the animal moves

in such a direction that the symmetrical elements of the surface of

the body are struck by light of the same intensity at the same angle,

so that as a consequence equal masses of photosensitive substances

are produced in symmetrical elements of their eyes or skin in equal

times. The effect on the symmetrical muscles will be identical. As

soon as one of the lights is a little stronger the animal will deviate

towards this light, in case it is positively heliotropic and towards

the weaker light if it is negatively heliotropic. This deviation again

is not the product of chance but follows a definite law as Patten[230]

has recently shown. He used the negatively heliotropic larvæ of the

blowfly. These larvæ were made to record their trail while moving under

the influence of the two lights. The results of the measurements of

2500 trails showing the progressive increase in angular deviation of

the larvæ (from the perpendicular upon the line connecting the two

lights), with increasing differences between the lights, are given in

the following table. Since the deviation or angular deflection of the

larvæ is towards the weaker of the two lights it is marked negative.

[230] Patten, Bradley M., _Am. Jour. Physiol._, 1915, xxxviii., 313.

TABLE VIII

-----------------------+-------------------------------

_Percentage Difference | _Average Angular Deflection of

in the Intensity | the Two Paths of the Larvæ

of the Two Lights_ | towards the Weaker Light_

-----------------------+-------------------------------

_Per Cent._ | _Degrees_

0 | -0.09

8-1/3 | -2.77

16-2/3 | -5.75

25 | -8.86

33-1/3 | -11.92

50 | -20.28

66-2/3 | -30.90

83-1/3 | -46.81

100 | -77.56

-----------------------+-------------------------------

Let us assume that the negatively heliotropic animal is at an equal

distance from the two unequal lights and placed so that at the

beginning of the experiment its median plane is at right angles to

the line connecting the two lights, but with its head turned away

from them. In that case the velocity of reaction in the symmetrical

photosensitive elements of the eyeless larvæ is greater on the side of

the stronger light. Since the animal is negatively heliotropic this

will result in a greater relaxation or a diminution of the energy

production of the muscles turning the head of the animal towards the

side of the stronger light. Hence the animal will automatically deviate

from the straight line towards the side of the weaker light. By the

alteration of the position of its body the photosensitive elements

exposed to the stronger of the two lights will be put at a less

efficient angle and hence the rate of photochemical reaction on this

side will be diminished. The deviation from the perpendicular in which

the animal will ultimately move will be such that as a consequence, the

rate of photochemical reaction in symmetrical elements is again equal.

The ultimate direction of motion will, according to our theory always

be such that the mass of chemical products formed under the influence

of light in symmetrical photosensitive elements during the same time is

equal.

Patten also investigated the question whether the same difference of

percentage between two lights would give the same deviation, regardless

of the absolute intensities of the lights used. The absolute intensity

was varied by using in turn from one to five glowers. The relative

intensity between the two lights varied in succession by 0, 8-1/3,

16-2/3, 25, 33-1/3, and 50 per cent. Yet the angular deflections were

within the limits of error identical for each relative difference of

intensity of the two lights no matter whether, 1, 2, 3, 4, or 5 glowers

were used. The following table shows the result.

TABLE IX

A TABLE BASED ON THE MEASUREMENTS OF 2700 TRAILS SHOWING THE ANGULAR

DEFLECTIONS AT FIVE DIFFERENT ABSOLUTE INTENSITIES