THE BEARERS OF THE HERITAGE
Before we can make any detailed analysis of the inheritance of characters
we should have some general idea of the physical structure of animals and
particularly some familiarity with the development of an individual from
the egg, as well as some knowledge of the nature of the germ-cells.
=The Cell the Unit of Structure.--=If we examine one of the higher
animals, as, for example, the horse, the dog, or man, we find that it is
made up of a large number of constituents, such as bones, muscles, nervous
elements, blood and other tissues. Each kind of tissue is composed of a
number of living units, ordinarily microscopic in size, which are known as
cells. A careful examination of various cells reveals that although they
may differ greatly in size, shape and minor details, they all alike
possess certain well-marked characteristics. Each when reduced to its
fundamental form is seen to consist of a small mass of living matter
termed protoplasm in which may usually be distinguished two regions--the
cell-body or _cytoplasm_, and the _nucleus_ (Fig. 3, p. 21). Any cell,
whether it be of the brain, of the liver, or from any organ of an animal
or plant, has this same fundamental structure. In addition, a limiting
membrane or wall of some kind is generally present, although it is not a
necessary constituent of all cells.
[Illustration: FIG. 3
Diagram of a cell showing various parts.]
=Unicellular Organisms.--=While such a structure as a tree or a horse is
composed of countless millions of cells, on the other hand numerous
organisms, both plant and animal, exist which consist of only one cell.
Yet this cell is just as characteristically a cell as are the components
of a complex animal or plant. It has the necessary parts, the cell body
and the nucleus. Moreover it exhibits all of the fundamental activities of
life, though in a simplified form, that a complex higher organism does.
=Importance of Cell-Theory.--=This discovery that every living thing is a
single cell or an aggregation of cooperating cells and cell-products is
one of our most important biological generalizations because it has
brought such a wide range of phenomena under a common point of view. In
the first place, the structure of both plants and animals is reducible to
a common fundamental unit of organization. Moreover, both physiological
and pathological phenomena are more readily understood since we recognize
that the functions of the body in health or disease are in large measure
the result of the activities of the individual cells of the functioning
part. Then again, the problems of embryological development have become
much more sharply defined since it could be shown that the egg is a single
cell and that it is through a series of divisions of this cell and
subsequent changes in the new cells thus formed that the new organism is
built up. And lastly, the problem of hereditary transmission has been
rendered more definite and approachable by the discovery that the male
germ is likewise a single cell, that fertilization of the egg is therefore
the union of two cells, and that in consequence the mechanism of
inheritance must be stowed away somehow in these two cells.
=Heredity in Unicellular Forms.--=In unicellular animals one can readily
see how it is possible for an individual always to give rise to its own
kind. One of the simplest of the single-celled animals is the _Ameba_
(Fig. 4, p. 24).
The ameba eats and grows as do other animals. Sooner or later it reaches a
size beyond which it can not increase advantageously, yet it is
continuously taking in new food material which stimulates it to further
growth. Here then is a problem. The ameba solves this difficulty by
dividing to form two amebae. Such a division is illustrated in Fig. 4, p.
24. First the nucleus divides, then the cell-body. When the two new amebae
separate completely each renews the occupation of eating and growing. But
what has become of the parent? Here, where once existed a large adult
ameba are two young amebae. The parent individual as such has disappeared,
yet there has been no death, for we have simply two bits of living jelly
in place of one. They will in turn repeat the same process, so will their
offspring, and thus, barring accident, this growth and reproduction, or
overgrowth as we may regard it, may go on forever, as far as we know. Here
the problem of heredity, or the resemblance of offspring to parent, is not
a very complicated one. The substance of the cell-body and cell-nucleus
divides into two similar halves, so that each descendant has the substance
of the parent in its own body, only it has but half as much. It differs
from the parent, not in quality or kind, but in size.
[Illustration: FIG. 4
Six successive stages in the division of _Ameba polypodia_ (after
Schulze). The nucleus is seen as a dark spot in the interior.]
=Reproduction and Heredity in Colonial Protozoa.--=There are enormous
numbers of these single-celled animals existing in all parts of the world.
Some are simple like the ameba, others are very complex in structure.
Many, after division, move apart and pursue wholly independent courses of
existence. On the other hand we find a modification appearing in some
which is of the greatest importance. After division instead of moving
apart the two cells may remain side by side and divide further to form
two more, these in turn may divide and thus the process goes on until
there is formed what is known as a colony. Each cell of such a colony
resembles the original ancestral cell because each is a part of the actual
substance of that cell. As in the ameba, the first two cells are the
ancestral cell done up in two separate packets, and thus finally the full
quota of cells must be so many separate packets of the same kind of
material. Inasmuch as each is but a repetition of its original ancestor,
it can, and at times does, produce a colony of the same kind as that
ancestor produced.
=Conjugation.--=At longer or shorter intervals, however, we find that two
individuals, on the disruption of the old colony, instead of continuing
the routine of establishing new colonies through a series of cell
divisions, very radically alter their behavior. They unite and fuse into a
single larger individual. This process is called _conjugation_. We find it
occurring even in some species of ameba. The conjugating cells in some
colonies are alike in size and appearance, in others different.
=Specialization of Sex-Cells.--=A beautiful sphere-shaped colony known as
_Volvox_ is to be found occasionally in roadside pools. Depending on the
species of _Volvox_ to which it belongs, the colony may be made up of from
a few hundred to several thousand individuals arranged in a single layer
about the fluid-filled center of the sphere and bound together by a clear
jelly-like inter-cellular substance. Each individual cell also connects
with its neighbors by means of thin threads of living matter. One of the
largest species is _Volvox globator_, one edge of which is represented in
Fig. 5, p. 27. Mutual pressure of the cells gives them a polygonal shape
when viewed from the surface. Each cell, with a few exceptions to be noted
immediately, bears two long flagella, whip-like structures which project
out into the water. The lashing of these flagella gives the ball a rotary
motion and thus it moves about. When the colony has reached its adult
condition and is ready to reproduce itself, certain cells without flagella
and somewhat larger than the ordinary cells become more rounded in outline
and increase considerably in size through the acquisition of food
materials. They are then known as egg cells or ova. Each ovum finally
enters on a series of cell-divisions forming a mass of smaller and smaller
cells which gradually assumes the form of a hollow sphere like the parent
colony. The young colonies thus formed drop into the interior of the
parent colony to escape later to the outside as independent swimming
organisms when the old colony dies and disintegrates.
=The Fertilized Ovum Termed a Zygote.--=After a number of generations of
such asexual reproduction, sexual reproduction occurs. The ova arise as
usual. Certain members of the colony, on the other hand, go to the other
extreme and divide up into bundles of from sixty-four to one hundred
twenty-eight minute slender cells, each provided with flagella for
locomotion. When mature these small flagellate cells, now known as
_spermatozoa_, escape into the interior of the parent colony and swim
about actively. Ultimately each ovum is penetrated by a spermatozoon, the
two cells fuse completely and thus form the single _fertilized ovum_ or
_zygote_. The body-cells of the mother colony finally disintegrate. After
a period of rest each zygote, through a series of cell-divisions, develops
into an adult Volvox. In some species of Volvox a still further advance is
seen, in that instead of both kinds of gametes being produced in the same
colony, the ova may be produced by one colony and the spermatozoa by
another. Here, then, we have the foreshadowings of two sexes as separate
individuals, a phenomenon of universal occurrence among the highest forms
of animal life.
[Illustration: FIG. 5
_Volvox globator_ (from Hegner after Oltmanns). Half of a sexually
reproducing colony: _o_, eggs; _s_, spermatozoa.]
=Advancement Seen in the Volvox Colony.--=In the Volvox colony there is a
distinct advance over the conditions met with in various lower protozoan
colonies in that only certain individuals of the colony take part in the
process of reproduction and these individuals are of two distinct types;
one is a larger, food-laden cell or egg and the other a small, active,
fertilizing cell. The motile forms are produced in much greater numbers
than the eggs, plainly because they have to seek the egg and many will
doubtless perish before this can be accomplished. This disparity in number
is only a means of insuring fertilization of the egg. The remaining cells
of the body carry on the ordinary activities of the colony such as
locomotion and nutrition and have ceased to take any part in the
production of new colonies.
=Natural Death Appears With the Establishment of a Body Distinct from the
Germ.--=Volvox is an organism of unusual interest because in it we see a
prophecy of what is to come. Although still regarded as a colony of
single-celled individuals, it represents in reality a transition between
the whole group of unicellular animals termed protozoa and the many celled
animals characterized by the possession of distinct tissues, known as
_Metazoa_. Moreover, it shows an interesting stage in the establishment of
a body or _soma_ distinct from special reproductive cells which have taken
on the function of reproducing the colony. In such colonial forms natural
death is found appearing for the first time, the reproductive cells alone
continuing to perpetuate the species. Then again Volvox represents an
important step in the establishment of sex in the animal kingdom for in
its sexual reproduction the conjugating cells known as _gametes_ are no
longer alike in appearance but have become differentiated into definite
ova and spermatozoa.
In Volvox as in the other organisms which we have studied we find that all
of the cells including the germ-cells are produced by the repeated
division of a parent cell, and consequently each must contain the
characteristic living substance of that parent. Many other forms might be
cited to illustrate reproduction in single-celled animals, whether free or
in colonies, but all such cases would be practically but repetitions or
modifications of those we have already examined.
=Specialization in Higher Organisms.--=If we pass on to the higher animals
and plants which are not single cells or colonies of similar cells but
organisms made up of many different kinds of cells, we find a pronounced
extension of the phenomenon met with in Volvox. Instead of each cell
executing independently all of the life relations, certain ones are set
apart for the performance of certain functions to the exclusion of other
functions which are carried on by other members of the aggregation. Thus
the organism as a whole has all the life relations carried on, but, as it
were, by specialists.
=Sexual Phenomena in Higher Forms.--=In the reproduction of multicellular
organisms, one sees likewise but a continuation of the phenomena exhibited
in Volvox. Ordinarily, each new form is produced by the successive
divisions of a single germ-cell which in the vast majority of cases has
conjugated with another germ-cell. In the development of the egg, as the
divisions proceed, groups of cells become modified for their particular
work until the entire organism is completed. During development certain
cells are set apart for reproduction of the form just as they were in
Volvox. These two kinds of reproductive cells in multicellular organisms
are derived ordinarily from two separate individuals known as male and
female, though there are some exceptions. The main difference between
these cells which will have to unite to form a single fertile germ-cell,
is that they have specialized in different directions; one is small and
active, the other large, food-laden and passive. But with two such
germ-cells coming as they do from two individuals, one the male, the other
the female, it is obvious that the actual living substance of which each
germ is composed will be distinctive of its own parental line and that
when the germs unite these distinctive factors commingle, hence the
complications of double ancestry arise.
=Structure of the Cell.--=Before we can understand certain necessary
details of the physical mechanism of inheritance we must inquire a little
further into the finer structure of the cell and into the nature of cell
division. A typical cell, as it would appear after treatment with various
stains which bring out the different parts more distinctly, is shown in
Fig. 3, p. 21. Typical, not that any particular kind of living cell
resembles it very closely in appearance, but because it shows in a
diagrammatic way the essential parts of a cell. In the diagram, there are
two well-marked regions; a central _nucleus_ and a peripheral cell-body or
_cytoplasm_. Other structures are pictured but only a few of them need
command our attention at present. At one side of the nucleus one observes
a small dot or granule surrounded by a denser area of cytoplasm. This body
is called the _centrosome_. The nucleus in this instance is bounded by a
well-marked nuclear membrane and within it are several substances. What
appear to be threads of a faintly staining material, the _linin_, traverse
it in every direction and form an apparent network. The parts on which we
wish particularly to rivet our attention are the densely stained
substances scattered along or embedded in the strands of this network in
irregular granules and patches. This substance is called _chromatin_. It
takes its name from the fact that it shows great affinity for certain
stains and becomes intensely colored by them. This deeply colored portion
of the cell, the chromatin, is by most biologists regarded as of great
importance from the standpoint of heredity. One or more larger masses of
chromatin or chromatin-like material, known as _chromatin nucleoli_, are
often present, and not infrequently a small spheroidal body, differing in
its staining reactions from the chromatin-nucleolus and sometimes called
the _true nucleolus_, exists.
=Cell-Division.--=In the simplest type of cell-division the nucleus first
constricts in the middle, and finally the two halves separate. This
separation is followed by a similar constriction and final division of the
entire cell-body, which results in the production of two new cells. This
form of cell-division is known as _simple_ or _direct division_. Such a
simple division, while found in higher animals, is less frequent and
apparently much less significant than another type of division which
involves profound changes and rearrangements of the nuclear contents. The
latter is termed _mitotic_ or _indirect_ cell-division. Fig. 6, p. 33,
illustrates some of the stages which are passed through in indirect
cell-division. The centrosome which lies passively at the side of the
nucleus in the typical cell (Fig. 6_a_, p. 33) awakens to activity,
divides and the two components come to lie at the ends of a fibrous
spindle. In the meantime, the interior of the nucleus is undergoing a
transformation. The granules and patches of chromatin begin to flow
together along the nuclear network and become more and more crowded until
they take on the appearance of one or more long deeply-stained threads
wound back and forth in a loose skein in the nucleus (Fig. 6_b_, p. 33).
If we examine this thread closely, in some forms it may be seen to consist
of a series of deeply-stained chromatin granules packed closely together
intermingled with the substance of the original nuclear network.
As the preparations for division go on the coil in the nucleus breaks up
into a number of segments which are designated as _chromosomes_ (Fig.
6_c_, p. 33). The nuclear membrane disappears. The chromosomes and the
spindle-fibers ultimately become related in such a way that the
chromosomes come to lie at the equator of the spindle as shown in Fig
6_d_, p. 33. Each chromosome splits lengthwise to form two daughter
chromosomes which then diverge to pass to the poles of the spindle (Figs.
6_e_ and _f_, p. 33). Thus each end of the spindle comes ultimately to be
occupied by a set of chromosomes. Moreover each set is a duplicate of the
other, because the substance of any individual chromosome in one group has
its counterpart in the other. In fact this whole complicated system of
indirect division is regarded by most biologists as a mechanism for
bringing about the precise halving of the chromosomes.
[Illustration: FIG. 6
Diagram showing representative stages in mitotic or indirect
cell-division: _a_, resting cell with reticular nucleus and single
centrosome; _b_, the two new centrosomes formed by division of the old one
are separating and the nucleus is in the spireme stage; _c_, the nuclear
wall has disappeared, the spireme has broken up into six separate
chromosomes, and the spindle is forming between the two centrosomes; _d_,
equatorial plate stage in which the chromosomes occupy the equator of the
spindle; _e_, _f_, each chromosome splits lengthwise and the daughter
chromosomes thus formed approach their respective poles; _g_,
reconstruction of the new nuclei and division of the cell body; _h_,
cell-division completed.]
The chromosomes of each group at the poles finally fuse and two new
nuclei, each similar to the original one, are constructed (Figs. 6_g_ and
_h_, p. 33). In the meantime a division of the cell-body is in progress
which, when completed, results in the formation of two complete new cells.
As all living matter if given suitable food, can convert it into living
matter of its own kind, there is no difficulty in conceiving how the new
cell or the chromatin material finally attains to the same bulk that was
characteristic of the parent cell. In the case of the chromatin, indeed,
it seems that there is at times a precocious doubling of the ordinary
amount of material before the actual division occurs.
=Chromosomes Constant in Number and Appearance.--=With some minor
exceptions, to be noted later, which increase rather than detract from the
significance of the facts, the chromosomes are always the same in number
and appearance in all individuals of a given species of plants or animals.
That is, every species has a fixed number which regularly recurs in all of
its cell-divisions. Thus the ordinary cells of the rat, when preparing to
divide, each display sixteen chromosomes, the frog or the mouse,
twenty-four, the lily twenty-four, and the maw-worm of the horse only
four. The chromosomes of different kinds of animals or plants may differ
very much in appearance. In some they are spherical, in others rod-like,
filamentous or perhaps of other forms. In some organisms the chromosomes
of the same nucleus may differ from one another in size, shape and
proportions, but if such differences appear at one division they appear at
others, thus showing that in such cases the differences are constant from
one generation to the next.
=Significance of the Chromosomes.--=The question naturally arises as to
what is the significance of the chromosomes. Why is the accurate
adjustment which we have noted for their division necessary? The very
existence of an elaborate mechanism so admirably adapted to their precise
halving, predisposes one toward the belief that the chromosomes have an
important function which necessitates the retention of their individuality
and their equal division. Many biologists accept this along with other
evidence as indicating that in chromatin we have a substance which is not
the same throughout, that different regions of the same chromosome have
different physiological values.
When the cell prepares for divisions, the granules, as we have seen,
arrange themselves serially into a definite number of strands which we
have termed chromosomes. Judging from all available evidence, the granules
are self-propagating units; that is, they can grow and reproduce
themselves. So that what really happens in mitosis in the splitting of the
chromosomes is a precise halving of the series of individual granules of
which each chromosome is constituted, or in other words each granule has
reproduced itself. Thus each of the two daughter cells presumably gets a
sample of every kind of chromosomal particle, hence, the two cells are
qualitatively alike. To use a homely illustration we may picture the
individual chromosomes to ourselves as so many separate trains of freight
cars, each car of which is loaded with different merchandise. Now, if
every one of the trains could split along its entire length and the
resulting halves each grow into a train similar to the original, so that
instead of one there would exist two identical trains, we should have a
phenomenon analogous to that of a dividing chromosome.
=Cleavage of the Egg.--=It is through a series of such divisions as these
that the zygote or fertilized egg-cell builds up the tissues and organs of
the new organism. The process is technically spoken of as _cleavage_.
Cleavage generally begins very shortly after fertilization. The fertile
egg-cell divides into two, the resulting cells divide again and thus the
process continues, with an ever-increasing number of cells.
=Chief Processes Operative in Building the Body.--=Although of much
interest, space will not permit of a discussion in detail of the building
up of the special organs and tissues of the body. It must suffice merely
to mention the four chief processes which are operative. These are, (1)
infoldings and outfoldings of the various cell complexes; (2)
multiplication of the component cells; (3) special changes (_histological
differentiation_) in groups of cells; and (4) occasionally resorption of
certain areas of parts.
=The Origin of the New Germ-Cells.--=On account of the unusual importance
from the standpoint of inheritance, which attaches to the germ-cells, a
final word must be said about their origin in the embryo. While the
evidence is conflicting in some cases, in others it has been well
established that the germ-cells are set apart very early from the cells
which are to differentiate into the ordinary body tissues. Fig. 7_A_, p.
38, shows a section through the eight-celled stage of _Miastor_, a fly, in
which a single large, primordial germ-cell (_p. g. c._) has already been
set apart at one end of the developing embryo. The nuclei of the rest of
the embryo still lie in a continuous protoplasmic mass which has not yet
divided up into separate cells. The densely stained nuclei at the opposite
end of the section are the remnants of nurse-cells which originally
nourished the egg. Fig. 7_B_, p. 38, is a longitudinal section through a
later stage in the development of _Miastor_; the primitive germ-cells
(ooeg) are plainly visible. Still other striking examples might be cited.
Even in vertebrates the germ-cells may often be detected at a very early
period.
=Significance of the Early Setting Apart of the Germ-Cells.--=It is of
great importance for the reader to grasp the significance of this early
setting apart of the germ-cells because so much in our future discussion
hinges on this fact. The truth of the statement made in a previous chapter
that the body of an individual and the reproductive substance in that body
are not identical now becomes obvious. For in such cases as those just
cited one sees the germinal substance which is to carry on the race set
aside at an early period in a given individual; it takes no part in the
formation of that individual's body, but remains a slumbering mass of
potentialities which must bide its time to awaken into expression in a
subsequent generation. Thus an egg does not develop into a body which in
turn makes new germ-cells, but body and germ-cells are established at the
same time, the body harboring and nourishing the germ-cells, but not
generating them (Fig. 2, p. 13). The same must be true also in many cases
where the earliest history of the germ-cells can not be visibly followed,
because in any event, in all higher animals, they appear long before the
embryo is mature and must therefore be descendants of the original
egg-cell and not of the functioning tissues of the mature individual. This
need not necessarily mean that the germ-cells have remained wholly
unmodified or that they continue uninfluenced by the conditions which
prevail in the body, especially in the nutritive blood and lymph stream,
although as a matter of fact most biologists are extremely skeptical as to
the probability that influences from the body beyond such general
indefinite effects as might result from under-nutrition or from poisons
carried in the blood, modify the intrinsic nature of the germinal
substances to any measurable extent.
[Illustration: FIG. 7
A--Germ-cell (_p. g. c._) set apart in the eight-celled stage of cleavage
in _Miastor americana_ (after Hegner). The walls of the remaining seven
somatic cells have not yet formed though the resting or the dividing (_M
p_) nuclei may be seen; _c R_, chromatin fragments cast off from the
somatic cells.
B--Section lengthwise of a later embryo of _Miastor_; the primordial
egg-cells (_ooeg__{3}) are conspicuous (after Hegner).]
=Germinal Continuity.--=The germ-cells are collectively termed the
_germinal protoplasm_ and it is obvious that as long as any race continues
to exist, although successive individuals die, some germinal protoplasm is
handed on from generation to generation without interruption. This is
known as the theory of _germinal continuity_. When the organism is ready
to reproduce its kind the germ-cells awaken to activity, usually
undergoing a period of multiplication to form more germ-cells before
finally passing through a process of what is known at _maturation_, which
makes them ready for fertilization. The maturation process proper, which
consists typically of two rapidly succeeding divisions, is preceded by a
marked growth in size of the individual cells.
=Individuality of Chromosomes.--=Before we can understand fully the
significance of the changes which go on during maturation we shall have to
know more about the conditions which prevail among the chromosomes of
cells. As already noted each kind of animal or plant has its own
characteristic number and types of chromosomes when these appear for
division by mitosis. In many organisms the chromosomes are so nearly of
one size as to make it difficult or impossible to be sure of the identity
of each individual chromosome, but on the other hand, there are some
organisms known in which the chromosomes of a single nucleus are not of
the same size and form (Fig. 8, p. 41). These latter cases enable us to
determine some very significant facts. Where such differences of shape and
proportion occur they are constant in each succeeding division so that
similar chromosomes may be identified each time. Moreover, in all ordinary
mitotic divisions where the conditions are accurately known, these
chromosomes of different types are found to be present as pairs of similar
elements; that is, there are two of each form or size.
=Pairs of Similar Chromosomes in the Nucleus Because One Chromosome Comes
from Each Parent.--=When we recall that the original fertilized egg from
which the individual develops is really formed by the union of two
gametes, ovum and spermatozoon, and that each gamete, being a true cell,
must carry its own set of chromosomes, the significance of the pairs of
similar chromosomes becomes evident; one of each kind has probably been
contributed by each gamete. This means that the zygote or fertile ovum
contains double the number of chromosomes possessed by either gamete, and
that, moreover, each tissue-cell of the new individual will contain this
dual number. For, as we have seen, the number of chromosomes is, with
possibly a few exceptions, constant in the tissue-cells and early
germ-cells in successive generations of individuals. For this to be true
it is obvious that in some way the nuclei of the conjugating gametes have
come to contain only half the usual number. Technically the tissue-cells
are said to contain the _diploid_ number of chromosomes, the gametes the
reduced or _haploid_ number.
[Illustration: FIG. 8
A--Chromosomes of the mosquito (_Culex_) after Stevens.
B--Chromosomes of the fruit-fly (_Drosophila_) after Metz.
Both of these forms have an unusually small number of chromosomes.]
=In Maturation the Number of Chromosomes Is Reduced by One-Half.--=This
halving, or as it is known, _reduction_ in the number of chromosomes is
the essential feature of the process of maturation. It is accomplished by
a modification in the mitotic division in which instead of each chromosome
splitting lengthwise, as in ordinary mitosis, the chromosomes unite in
pairs (Fig. 9_b_, p. 42), a process known technically as _synapsis_, and
then apparently one member of each pair passes entire into one new
daughter cell, the other member going to the other daughter cell (Fig.
9_c_, p. 42). In the pairing preliminary to this _reduction division_,
leaving out of account certain special cases to be considered later,
according to the best evidence at our command the union always takes place
between two chromosomes which match each other in size and appearance.
Since one of these is believed to be of maternal and the other of
paternal origin, the ensuing division separates corresponding mates and
insures that each gamete gets one of each kind of chromosome although it
appears to be a matter of mere chance whether or not a given cell gets the
paternal or the maternal representative of that kind.
[Illustration: FIG. 9
Diagram to illustrate spermatogenesis: _a_, showing the diploid number of
chromosomes (six is arbitrarily chosen) as they occur in divisions of
ordinary cells and spermatogonia; _b_, the pairing (synapsis) of
corresponding mates in the primary spermatocyte preparatory to reduction;
_c_, each secondary spermatocyte receives three, the haploid number of
chromosomes; _d_, division of the secondary spermatocytes to form _e_,
spermatids, which transform into _f_, spermatozoa.]
=Maturation of the Sperm-Cell.--=In the maturation of the male gamete the
germ-cell, now known as a _spermatogonium_, increases greatly in size to
become a _primary spermatocyte_. In each primary spermatocyte the pairing
of the chromosomes already alluded to occurs as indicated in Fig. 9_b_, p.
42, where six is taken arbitrarily to indicate the ordinary or _diploid_
number of chromosomes, and three the reduced or _haploid_ number. The
division of the primary spermatocyte gives rise to two _secondary
spermatocytes_ (_c_), the paired chromosomes separating in such a way that
a member of each pair goes to each secondary spermatocyte. Each secondary
spermatocyte (_d_) soon divides again into two _spermatids_ (_e_), but in
this second division the chromosomes each split lengthwise as in an
ordinary division so that there is no further reduction. In some forms the
reduction division occurs in the secondary spermatocytes instead of the
primary. Each spermatid transforms into a mature spermatozoon (_f_). The
spermatozoa of most animals are of linear form, each with a head, a
middle-piece and a long vibratile tail which is used for locomotion. The
head consists for the most part of the transformed nucleus and is
consequently the part which bears the chromosomes.
=Maturation of the Egg-Cell.--=As regards the behavior of the chromosomes
the maturation of the ovum parallels that of the sperm-cell. There are not
so many primordial germ-cells formed and only one out of four of the
ultimate cells becomes a functional egg. As in maturation of the
sperm-cell there is a growth period in which _ooegonia_ enlarge to become
_primary ooecytes_ (Fig. 10_b_, p. 45). In each primary ooecyte as in the
primary spermatocyte the chromosomes pair and two rapidly succeeding
divisions follow in one of which the typical numerical reduction in the
chromosomes occurs. A peculiarity in the maturation of the ovum is that
there is a very unequal division of the cytoplasm in cell division so that
three of the resulting cells usually termed _polar bodies_ are very small
and appear like minute buds on the side of the fourth or egg-cell proper.
The scheme of this formation of the polar bodies is indicated in Fig. 10,
p. 45. In Fig. 10_b_ the chromosomes are seen paired and ready for the
first division; that is, for the formation of the first polar body. Figs.
10_c_, _d_, p. 45, show the giving off of this body. Note that while only
a small proportion of the cytoplasm passes into this tiny cell, its
chromatin content is as great as that of the ovum. A second polar body
(Figs. 10_e_, _f_, p. 45) is formed by the egg, but in this case each
chromosome splits lengthwise, as in ordinary mitosis, and there is no
further numerical reduction. In the meantime, typically, a third polar
body is formed by division of the first. (Stages _e_, _f_, _g_.)
=Parallel Between the Maturation of Sperm- and Egg-Cell.--=This rather
complex procedure of the germ-cells will be rendered more intelligible
through a careful study of Figs. 9 and 10, pp. 42 and 45, and Fig. 11, p.
46, which indicates the parallel conditions in spermotogenesis and
ooegenesis.
[Illustration: FIG. 10
Diagram to illustrate ooegenesis: _a_, showing the diploid number of
chromosomes (six is arbitrarily chosen) as they occur in ordinary cells
and ooegonia; _b_, the pairing of corresponding mates preparatory to
reduction; _c_, _d_, reduction division, giving off of first polar body;
_e_, egg preparing to give off second polar body, first polar body ready
for division; _f_, _g_, second polar body given off, division of first
polar body completed. The egg nucleus, now known as the female pronucleus,
and each body contain the reduced or haploid number of chromosomes.]
The view now generally held regarding the polar bodies is that they are
really abortive eggs. They later disappear, taking no part in
embryo-formation. It can readily be seen how such an unequal division is
advantageous to the large cell, for it receives all of the rich store of
food material that would be distributed among the four cells if all were
of equal size. This increased amount of food is a favorable provision for
the forthcoming offspring whose nourishment is thus more thoroughly
insured.
[Illustration: FIG. 11
Diagram showing the parallel between maturation of the sperm-cell and
maturation of the ovum.]
On the other hand, all of the sperm-cells develop into complete active
forms, which, as aforesaid, usually become very much elongated and develop
a motile organ of some kind. In such cells an accumulation of food to any
large extent would hinder rather than help them, because it would
seriously interfere with their activity.
=Fertilization.--=In fertilization (Fig. 12, p. 48) the spermatozoon
penetrates the wall of the ovum and after undergoing considerable
alteration its nucleus fuses with the nucleus of the egg. In some forms
only the head (nucleus) and middle-piece enter, the tail being cut off by
a so-called fertilization membrane which forms at the surface of the egg
and effectually blocks the entrance of other spermatozoa. Thus normally
only one spermatozoon unites with an egg. In some forms while several may
enter the egg only one becomes functional. As soon as the nucleus of the
spermatozoon, now known as the male _pronucleus_, reaches the interior of
the egg, it enlarges and becomes similar in appearance to the female
_pronucleus_. It swings around in such a way (Fig. 12_b_, p. 48) that the
middle piece, now transformed into a centrosome, lies between it and the
female pronucleus. The two pronuclei (_c_, _d_, _e_), each containing the
reduced number of chromosomes, approach, the centrosome divides, the
nuclear walls disappear, the typical division spindle forms, and the
chromosomes of paternal and maternal origin respectively come to lie side
by side at the equator of the spindle ready for the first division or
cleavage (_f_, _g_). It will be noted that the individual chromosomes do
not intermingle their substance at this time, but that each apparently
retains its own individuality. There is considerable evidence which
indicates that throughout life the chromosomes contributed by the male
parent remain distinct from those of the female parent. Inasmuch as each
germ-cell, after maturation, contains only half the characteristic number
of chromosomes, the original number is restored in fertilization.
[Illustration: FIG. 12
Diagram to illustrate fertilization; [male], male pronucleus; [female],
female pronucleus; observe that the chromosomes of maternal and paternal
origin respectively do not fuse.]
=Significance of the Behavior of the Chromosomes.--=The question confronts
us as to what is the significance of this elaborate system which keeps the
chromosomes of constant size, shape and number; which partitions them so
accurately in ordinary cell-divisions; and which provides for a reduction
of their numbers by half in the germ-cell while yet securing that each
mature gamete gets one of each kind of chromosome. Most biologists look on
these facts as indicating that the chromosomes are specifically concerned
in inheritance.
In the first place it is recognized that as regards the definable
characters which separate individuals of the same species, offspring may
inherit equally from either parent. And it is a very significant fact that
while the ovum and spermatozoon are very unequal in size themselves, the
chromosomes of the two germ-cells are of the same size and number. This
parity in chromosomal contribution points clearly to the means by which an
equal number of character-determiners might be conveyed from each parent.
Moreover it is mainly the nucleus of the sperm-cell in some organisms
which enters the egg, hence the determiners from the male line must exist
wholly or largely somewhere in the nucleus. And the bulk of the nucleus in
the spermatozoon consists of the chromosomes or their products.
=A Single Set of Chromosomes Derived from One Parent Only Is Sufficient
for the Production of a Complete Organism.--=That a single or haploid set
of chromosomes as seen in the gametes is sufficient contribution of
chromatin for the production of a complete organism is proved by the fact
that the unfertilized eggs of various animals (many echinoderms, worms,
mollusks, and even the frog) may be artificially stimulated to development
without uniting at all with a spermatozoon. The resulting individual is
normal in every respect except that instead of the usual diploid number it
has only the single or haploid number of chromosomes. Its inheritance of
course is wholly of maternal origin. The converse experiment in
echinoderms in which a nucleus of male origin (that is, a spermatozoon)
has been introduced into an egg from which the original nucleus has been
removed shows that the single set of chromosomes carried by the male
gamete is also sufficient to cooperate with the egg-cytoplasm in
developing a complete individual.
=The Duality of the Body and the Singleness of the Germ.--=Since every
maternal chromosome in the ordinary cell has an equivalent mate derived
from the male parent, it follows therefore, supposing the chromosomes do
have the significance in inheritance attributed to them, that as regards
the measurable inheritable differences between two individuals, the
ordinary organism produced through the union of the two germ-cells is,
potentially at least, dual in nature. On the other hand through the
process of reduction the gametes are provided with only a single set of
such representatives. This duality of the body and singleness of the
mature germ is one of the most striking facts that come to light in
embryology. How well the facts fit in with the behavior of certain
hereditary characters will be seen later in our discussions of Mendelism.
=The Cytoplasm Not Negligible in Inheritance.--=Just what part is played
by the cytoplasm in inheritance is not clear, but it is probably by no
means a negligible one. The cytoplasm of a given organism is just as
distinctive of the species or of the individual of which it forms a part
as are the chromosomes. It is well established that neither nucleus nor
cytoplasm can fully function or even exist long without the other, and
neither can alone produce the other. They undoubtedly must cooperate in
building up the new individual, and the cytoplasm of the new individual is
predominantly of maternal origin. It is obvious that all of the more
fundamental characters which make up an organism, such, for instance, as
make it an animal of a certain order or family, as a human being or a dog
or a horse, are common to both parents, and there is no way of measuring
how much of this fundamental constitution comes from either parent, since
only closely related forms will interbreed. In some forms, moreover, the
broader fundamental features of embryogeny are already established before
the entrance of the spermatozoon. It is probable therefore that instead of
asserting that the entire quota of characters which go to make up a
complete individual are inherited from each parent equally, we are
justified only in maintaining that this equality is restricted to those
measurable differences which veneer or top off, as it were, the
individual. We may infer that in the development of the new being the
chromosomes of the egg together with those derived from the male work
jointly on or with the other germinal contents which are mostly
cytoplasmic materials of maternal origin.
=The Chromosomes Possibly Responsible for the Distinctiveness of Given
Characters.--=It seems probable that in the establishment of certain basic
features of the organism the cooperation of the cytoplasm with chromatin
of either maternal or paternal origin might accomplish the same end, but
that certain distinctive touches are added or come cumulatively into
expression through influences carried, predominantly at least, in the
chromatin from one as against the other parent. These last distinctive
characters of the plant or animal constitute the individual differences of
such organisms. In this connection it is a significant fact that in young
hybrids between two distinct species the early stages of development,
especially as regards symmetry and regional specifications, are
exclusively or predominantly maternal in character, but the male influence
becomes more and more apparent as development progresses until the final
degree of intermediacy is attained.
From the evidence at hand this much seems sure, that the paternal and
maternal chromosomes respectively carry substances, be they ferments,
nutritive materials or what not, that are instrumental in giving the final
parity of personal characters which we observe to be equally heritable
from either line of ancestry. It is clear that most of the characters of
an adult organism can not be merely the outcome of any unitary substance
of the germ. Each is the product of many cooperating factors and for the
final outcome any one cooperant is probably just as important in its way
as any other. The individual characters which we juggle to and fro in our
breeding experiments seem apexed, as it were, on more fundamental features
of organic chemical constitution, polarity, regional differentiation, and
physiological balance, but since such individual characters parallel so
closely the visible segregations and associations which go on among the
chromosomes of the germ-cells it would seem that they, at least, are
represented in the chromosomes by distinctive cooperants which give the
final touch of specificity to those hereditary characters which can be
shifted about as units of inheritance.
=Sex and Heredity.--=Whatever the origin of fertilization may have been in
the world of life, or whatever its earliest significance, the important
fact remains that to-day it is unquestionably of very great significance
in relation to the phenomena of heredity. For in all higher animals, at
least, offspring may possess some of the characteristics originally
present in either of two lines of ancestry, and this commingling of such
possessions is possible only through sexual reproduction. As has already
been seen, in the pairing of chromosomes previous to reduction, the
corresponding members of a pair always come together so that in the final
segregation each gamete is sure to have one of each kind although whether
a given chromosome of the haploid set is of maternal or paternal origin
seems to be merely a matter of chance. Thus, for instance, if we
arbitrarily represent the chromosomes of a given individual by _ABC_
_abc_, and regard _A_, _B_ and _C_ as of paternal and _a_, _b_ and _c_ as
of maternal origin, then in synapsis only _A_ and _a_ can pair together,
_B_ and _b_ and _C_ and _c_, but each pair operates independently of the
other so that in the ensuing reduction division either member of a pair
may get into a cell with either member of the other pairs. That is, the
line up for division at a given reduction might be any one of the
following, ABC/abc ABc/abC Abc/aBC AbC/aBc. This would yield the following
eight kinds of gametes, _ABC_, _abc_, _ABc_, _abC_, _Abc_, _aBC_, _AbC_,
_aBc_, each bearing one of each kind of chromosome required to cover the
entire field of characters necessary to a complete organism. And since
each sex would be equally likely to have these eight types of gametes and
any one of the eight in one individual might meet any one of the eight of
the other, the possible number of combinations in the production of a new
individual from such germ-cells would be 8x8, or 64. With the larger
numbers of chromosomes which exist in most animals it is readily seen that
the number of possible combinations becomes very great. Thus any
individual of a species with twenty chromosomes--and many animals,
including man, have more--would have ten pairs at the reduction period and
could therefore form (2)^{10}, or 1,024 different gametes in each sex. And
since any one of these in one sex would have an equal chance of meeting
with any one in the opposite sex, the total number of possible different
zygotes that might be produced would be (1,024)^{2}, or 1,048,576. Sex
therefore, through recombinations of ancestral materials, undoubtedly
means, among other things, the production of great diversity in
offspring.
DETERMINATION OF SEX
=Many Theories.--=From earliest times the problem of sex determination has
been one of keen interest, and needless to say hundreds of theories have
been propounded to explain it. Geddes and Thomson say that Drelincourt
recorded two hundred sixty-two so-called theories of sex production and
remark that since his time the number has at least been doubled. The
desirability of controlling sex has naturally appealed strongly to
breeders of domesticated animals.
A study of animals born in litters, or of twins, is enough in itself to
make us skeptical of theories of sex-determination based on nutritional or
external factors. In a litter of puppies, for example, there are usually
both males and females, although in their prenatal existence they have all
been subject to the same nutritional and environmental conditions.
Likewise in ordinary human twins one may be a boy, the other a girl,
whereas if the nutritional condition of the mother were the fact
determining sex, both should be boys or both girls. However, there are
twins known as _identical twins_ who are remarkably alike and who are
always of the same sex. But there is reason to suppose that identical
twins in reality come from the same zygote. Presumably in early
embryogeny, probably at the two-celled stage of cleavage, the two
blastomeres become separated and each gives rise to a complete individual
instead of only the half of one it would have produced had the two
blastomeres remained together. Such twins are monochorial; that is, they
grow inside the same fetal membrane, whereas each ordinary twin has its
own fetal membrane and has obviously originated from a separate ovum. It
has been established experimentally in several kinds of animals that early
cleavage blastomeres when isolated can each develop into a complete
individual. In man, ordinary twins are no more alike than ordinary
brothers and sisters, but identical twins are strikingly similar in
structure, appearance, habits, tastes, and even susceptibility to various
maladies. The fact that they are invariably of the same sex is a strong
reason for believing that sex was already developed in the fertile ovum
and consequently in the resulting blastomeres from that ovum.
The young of the nine-banded armadillo in a given litter are invariably of
the same sex and are closely similar in all features. Newman and Patterson
have shown that all the members of a litter come from the same egg.
Patterson has established the fact that cleavage of the egg takes place in
the usual manner, but later separate centers of development appear in the
early embryonic mass and give rise to the separate young individuals.
Again in certain insects where one egg indirectly gives rise to a chain of
embryos, or to a number of separate larvae, possibly as many as a thousand,
all of the latter are of the same sex. Even in some plants researches have
shown that sex is already determined at the beginning of development.
Then, too, much evidence has come to light recently showing that
sex-characters in certain cases behave as heritable characters and are
independent of external conditions. Lastly there is visible and convincing
evidence obtainable through microscopical observations that sex is
determined by a mechanism in the germ-cells themselves. It is chiefly to
these latter facts that I wish to direct attention for the present.
=The Sex Chromosome.--=The evidence centers about a special chromosome or
chromosome-group commonly designated as the _sex-chromosome_ or
_X-element_, which has been found in various species of animals, including
man. In the males of such animals this chromosome is present in addition
to the regular number of pairs, thus giving rise to an _uneven_ instead of
the conventional even number of chromosomes. This element remains
undivided in one of the maturation divisions of the spermatocytes, in some
forms in the first in others in the second, and passes entire to one pole
of the spindle (Fig. 13, p. 58). This results in the production of two
classes of cells, one containing the _X_-element and one not. The outcome
is that two corresponding classes of spermatozoa are produced. The
phenomena involved are diagrammatically represented in Fig. 13. It has
been clearly demonstrated in several cases that eggs fertilized by
spermatozoa which possess this _X_-element, always become females, those
fertilized by spermatozoa which do not possess it always develop into
males.
[Illustration: FIG. 13
Diagram illustrating the behavior of the _x_-element or sex-chromosome in
the maturation of the sperm-cell. In one of the two maturation divisions
(represented here as in the first) it passes undivided to one pole (_a_,
_b_, _c_), in the other it divides. Since the cell without the _x_-element
also divides the result is that ultimately from the original primary
spermatocyte (_a_) four cells are formed (_f_), two with the _x_-element
and two without it. Half of the spermatozoa therefore will bear an
_x_-element, half will be without it. In _a_ the ordinary chromosomes,
arbitrarily indicated as 10, are supposed to have already paired for
reduction so that the original diploid number in spermatogonia and
body-cells of the male would be 20 plus the _x_-chromosome.]
It has been found, furthermore, that in species in which the males possess
this extra element the females have two of them. That is, if the original
number in the somatic cells of the male were twenty-three, twenty-two
ordinary and one _X_-element, the number in the somatic cells of the
female would be twenty-four, or twenty-two ordinary and two _X_-elements.
It has been found that when the chromosomes of the female pair for the
reduction division, each chromosome uniting with its corresponding fellow,
the two _X_-elements in the female pair in the usual way so that every
egg-cell possesses an _X_-element. Thus every mature egg has an
_X_-element, while only half of the spermatozoa have one. That is, if we
assume twenty-three as the diploid number present originally in the
somatic cells of the male and twenty-four as the number in the female,
then one-half the spermatozoa of the male would contain the haploid number
eleven, and the other half, the number twelve, whereas every mature ovum
would contain twelve. Since there are equal numbers of the spermatozoa
with the _X_-element and without it, and inasmuch as presumably under
ordinary conditions one kind is as likely to fertilize the egg as the
other, then there are equal chances at fertilization of producing a zygote
with two _X_-elements or with but one.
Thus, Spermatozoon + _X_ by Ovum + _X_ = Zygote + _XX_.
Spermatozoon (no _X_) by Ovum + _X_ = Zygote + _X_.
We have already seen that the former is always female, the latter male. It
thus becomes possible to distinguish the sex of an embryo by counting the
chromosomes of its cells. This has been accomplished in several cases.
In some instances[1] the conditions may be much more complex than the
ones indicated--too complex in fact to warrant detailed discussion in an
elementary exposition--but the principle remains the same throughout, the
very complexity when thoroughly understood, strengthening rather than
weakening the evidence. In a few forms an interesting reversal of
conditions has been found in that the eggs instead of the spermatozoa show
the characteristic dimorphism.
Just what the exact relationship between sex-differentiation and the
_X_-element is has never been clearly established. It is possible that
this element is an actual sex-determinant, in the ordinary cases one
_X_-element determining the male condition and two _X_-elements producing
the female condition. On the other hand it might be argued that it is not
the determining factor but the expression of other cell activities which
do determine sex; that is, a sex accompaniment. Or again, it may be one of
several essential factors which must cooperate to determine sex.
SEX-LINKED CHARACTERS
The discovery of the remarkable behavior of certain characters in heredity
which can only be plausibly explained by supposing that they are linked
with a sex-determining factor still further strengthens our belief in the
existence of such a definite factor. Such characters are commonly termed
sex-linked characters.
=Sex-Linked Characters in Man.--=Since there are a number of them in man
we may choose one of these, such as color-blindness, for illustration. The
common form of color-blindness known as Daltonism in which the subject
can not distinguish reds from greens, a condition which seems to be due to
the absence of something which is present in individuals of normal color
vision, is far commoner in men than in women. Its type of inheritance,
sometimes termed "crisscross" heredity, has been likened to the knight
moves in a game of chess. The condition is transmitted from a color-blind
man through his daughter to half of her sons. Or, to go more into detail,
a color-blind father and normal mother have only normal children whether
sons or daughters. The sons continue to have normal children but the
daughters, although of normal vision themselves, transmit color-blindness
to one-half of their own sons. If such a woman marries a color-blind man,
as might easily happen in a marriage between cousins, then as a rule
one-half her daughters as well as one-half her sons will be color-blind.
[Illustration: FIG. 14
Diagram illustrating the inheritance of a sex-limited character such as
color-blindness in man on the assumption that the factor in question is
located in the sex-chromosome (from Loeb after Wilson). The normal
sex-chromosome is indicated by a black =X=, the one lacking the factor for
color perception, by a light X. It is assumed that a normal female is
mated with a color-blind male.]
In such cases what appears to be a mysterious procedure becomes very
simple if we assume that the defective character is associated with the
sex-determining factor, or to make it concrete let us say with the
_X_-element. The chart shown in Fig. 14, p. 62, indicates what the
germinal condition would be under the circumstances. The column to the
right represents the maternal, the one to the left the paternal line.
Since two _X_ means female and one _X_ male, and inasmuch as we have
assumed that the physical basis of the defect to which color-blindness is
due is conveyed by the _X_-element, we may represent the defective single
_X_ of the male in outline only (see first row). It is obvious that after
the reduction divisions (second row) the mature sex-cells of the female
will each contain a single normal _X_, the corresponding sex cells of the
male will contain either no _X_ or a defective _X_. Since if any member of
the class of spermatozoa containing no _X_, fertilizes an egg the
resulting zygote (row three) will have but one _X_ and that a normal one,
the individual which develops from the zygote will be normal as regards
color vision and moreover will be male because the condition one _X_
always means maleness. On the other hand, if any member of the class of
spermatozoa containing the defective _X_ fertilizes an egg two
_X_-elements are brought together and this of itself means femaleness. In
this case one of the _X_-elements is defective but the single normal _X_
is sufficient in itself to produce normal color vision. But when it comes
to the maturation of the sex-cells of this female, the pair of
_X_-elements are separated in the usual way with the result that half of
the mature ova contain a normal _X_ and half a defective _X_ (row four).
Since in a normal male, however, the mature reproductive cells will
contain either a normal _X_ or no _X_ (fourth row), any one of four
different kinds of matings may result. A sex-cell carrying normal _X_ of
the male may combine with an ovum containing normal X producing a normal
female (row five). Or such a cell may combine with an ovum carrying the
defective _X_, also producing a female but one who although of normal
color vision herself, like her mother, is a carrier of the defect. On the
other hand, any one of the spermatozoa without an _X_ may combine with an
ovum containing the normal _X_, in which case a normal male is produced
and, moreover, one who, like his mother's brothers, is incapable of
transmitting the defect. However, the sperm-cell devoid of an _X_ is just
as likely to fertilize an ovum carrying the defective _X_, in which event
the resulting individual, a male, must be color-blind because he contains
the defective _X_ alone. In other words, the chances are that one-half the
sons of a woman whose father was color-blind will be color-blind, the
other half perfectly normal; and that all of the daughters will be of
normal color vision although one-half of them will probably transmit the
defect to one-half of their sons. From a glance at the diagram it is
readily seen also that a color-blind female could result from the union of
a color-blind man (see first row) and the daughter of a color-blind man
(see third row). For half of the gametes of such a female would bear the
defect as would also that half of the gametes of the male which carry _X_,
hence the expectation would be that half of the daughters of such a union
would be color-blind and half would be carriers of color-blindness; and
that half of the sons would be color-blind and half normal. All the sons
of a color-blind woman would be color-blind because she has only defective
_X_-elements to pass on.
The inheritance of various other conditions in man follows more or less
accurately the same course as color-blindness. Among these may be
mentioned: _hemophilia_, a serious condition in which the blood will not
clot properly, thus rendering the affected individual constantly liable to
severe or fatal hemorrhage; near-sightedness (_myopia_) in some cases; a
degenerative disease of the spinal cord known as _multiple sclerosis_;
progressive atrophy of the optic nerve (_neuritis optica_); Gower's
_muscular atrophy_; some forms of _night-blindness_; in some cases
_ichthyosis_, a peculiar scaly condition of the skin. In one of my own
tabulations of a case of inheritance of "webbed" digits or _syndactyly_, a
condition in which two or more fingers or toes are more or less united, a
sex-linked inheritance is clearly indicated (Fig. 15), although from the
pedigrees recorded by other investigators this condition usually appears
in some of both the sons and daughters of an affected individual.
[Illustration: FIG. 15
Chart showing the inheritance of a case of syndactyly after the manner of
a sex-linked character. The affected individuals are represented in black;
squares indicate males, circles females. The condition is seen to be
inherited by males through unaffected females.]
=The Occurrence of Sex-Linkage in Lower Forms Renders Experiments
Possible.--=The course followed by such characters in man can be inferred
only from the pedigrees we can obtain from family histories. Fortunately,
however, such sex-linkage also occurs in lower animals and we are able
therefore to verify and extend our observations by direct experiments in
breeding. Several sex-linked characters have been found to exist in a
small fruit-fly known as _Drosophila_. Extensive breeding experiments with
this fly by Professor T. H. Morgan and his pupils have borne out
remarkably the interpretation that the characters in question are really
linked with a sex-determining factor.