THE BEGINNINGS OF CAUSAL MORPHOLOGY
Until well into the 'eighties animal morphology remained a purely
descriptive science, content to state and summarise the relations
between the coexistent and successive form-states of the same and of
different animals. No serious attempt had been made to discover the
causes which led to the production of form in the individual and in the
race.
It is true that evolution-theory had offered a simple solution of the
great problem of the unity in diversity of animal forms, but this
solution was formal merely, and went little beyond that abstract
deduction of more complex from simpler forms, which had been the main
operation of pre-evolutionary morphology. Little was known of the actual
causes of ontogeny, and nothing at all of the causes of phylogeny; it
was, for instance, mere rhetoric on Haeckel's part to proclaim that
phylogeny was the mechanical cause of ontogeny.
Animal physiology, on its side, had developed in complete isolation from
morphology into a science of the functioning of the adult and finished
animal, considered as a more or less stable physico-chemical mechanism.
Since the days of Ludwig, Claude Bernard and E. du Bois Reymond, the
physiologists' chief care had been to analyse vital activities into
their component physical and chemical processes, and to trace out the
interchange of matter and energy between the organism and its
environment. Physiologists had left untouched, perhaps wisely, the much
more difficult problem of the causes of the development of form. For all
practical purposes they took the animal-machine as given, and did not
trouble about its mode of origin. They held indeed that form-production
was due to a complex of physico-chemical causes, which they hoped some
day to unravel;[464] but this future physiology of development remained
quite embryonic.
Physiology then had not really come into contact with the problems of
form, and it could give the morphologist no direct help when he turned
to investigate the causes of form-production. It had, however, a
determining influence upon the methods of those who first broke ground
in this No Man's Land between morphology proper and physiology. But it
is significant that it was a morphologist and not a physiologist that
did the first spade-work.
The pioneer in this field, both as investigator and as thinker, was W.
Roux, who sketched in the 'eighties the main outlines of a new science
of causal morphology, to which he gave the name of
_Entwicklungsmechanik_. The choice of name was deliberate, and the word
implied, first, that the new science was essentially an investigation of
the development of form, not of the mode of action of a formed
mechanism, and second, that the methods to be adopted were
mechanistic.[465]
Though Roux was the only begetter of the science of
_Entwicklungsmechanik_, he was, of course, not the first to investigate
experimentally the formative processes of animal life. Study of
regeneration dates back to Trembley (1740-44), Reaumur (1742), Bonnet
(1745), and Spallanzani (1768-82),[466] and in the years preceding Roux's
activity good work was done by Philipeaux. A beginning had been made
with experimental teratology by E. Geoffroy St Hilaire and others, and
the work of C. Dareste[467] remains classical. Back in the 18th century,
some of John Hunter's experiments had a bearing upon the problems of
form; his work on transplantation was followed up in the 19th century by
Flourens, P. Bert, Ollier and many others. In founding in 1872 the
_Archives de Zoologie experimentale et generale_ H. de Lacaze-Duthiers
put forward in his introduction a powerful plea for the use of the
experimental method in zoology.
In some ways more directly connected with _Entwicklungsmechanik_ was
His's attempt in 1874[468] to explain on mechanical principles the
formation of certain of the embryonic organs by the bendings and
foldings of tubes or plates of cells. "His compared the various layers
of the chick embryo to elastic plates and tubes; out of these he
suggested that some of the principal organs might be moulded by mere
local inequalities of growth--the ventricles of the brain, for instance,
the alimentary canal, the heart--and he further succeeded in imitating
the formation of these organs by folding, pinching, and cutting
india-rubber tubes and plates in various ways."[469]
But Roux was undoubtedly the first to make a systematic survey of the
problems to be solved and to work out an organised method of attack. His
earliest work deals with the important problem of functional
adaptation--its importance to the organism, and its possible mechanistic
explanation. The first paper[470] was a study of the branching and
distribution of the arteries in the human body (1878), and a second
paper on the same subject followed in 1879.[471]
In these papers Roux showed how the development of the blood-vascular
system was largely determined by direct adaptation to functional
requirements, and he inferred the existence in the vascular tissues of
certain vital properties, in virtue of which the functional adaptation
of the blood-vessels came about. Thus the intima or inner lining must
possess the faculty of so reacting to the friction set up by the
blood-current as to oppose the least possible resistance to its flow;
the muscular coats must react to increased pressure by growing thicker,
and so on.
These papers were followed in 1881 by his well-known book, _Der Kampf
der Theile im Organismus_, which contained the working-out of his
mechanistic explanation of functional adaptation, and most of the
elements of his general "causal-analytical" theory of form production.
The significance of the book was popularly considered at the time to lie
in its supposed application of the selection idea to the explanation of
the internal adaptedness of animal structure--in the theory of "cellular
selection," and the book owed its success to its fitting in so well with
the prevalent Darwinism of the day. But its real importance, as a big
step towards causal morphology, was naturally not so fully appreciated.
During the next few years Roux continued his studies on functional
adaptation,[472] and at the same time made a new departure by
inaugurating, almost contemporaneously with the physiologist Pflueger,
the study of experimental embryology. Isolated observations had
previously been made upon the development of single blastomeres or parts
of blastulae, by Haeckel and Chun for instance,[473] but Roux[474] and
Pflueger[475] were the first to investigate the subject systematically,
choosing for their work the egg of the frog.[476] Roux continued for many
years to follow up this line of work.[477]
In 1890 he drew up a programme and manifesto[478] of
_Entwicklungsmechanik_ as "an anatomical science of the future," and in
1895 he founded the famous _Archiv fuer Entwicklungsmechanik_,[479]
publishing in the same year the two large volumes of his collected
papers,[480] of which the first volume dealt with functional adaptation,
the second with experimental embryology.
His subsequent work includes several important general papers;[481]
besides a number of special memoirs dealing with the factors of
development, and with his original subject, functional adaptation.[482]
In our sketch of his views we shall have occasion to refer particularly
to his publications of 1881, 1895 (the _Einleitung_), 1902, 1905, and
1910.
Although Roux's biological philosophy is out-and-out mechanistic, he yet
recognises the difficulty, even the impossibility, of straightway
reducing development to the physico-chemical level. He tries to steer a
course midway between the simplicist conceptions of the materialists and
the "metaphysics" of the neo-vitalist school, which the experimental
study of development and regeneration soon brought into being. In 1895
he writes:--"The too simple mechanistic conception on the one hand, and
the metaphysical conception on the other represent the Scylla and
Charybdis, between which to sail is indeed difficult, and so far by few
satisfactorily accomplished; it cannot be denied that with the increase
of knowledge the seduction of the second has lately notably increased"
(p. 23).
The _via media_ adopted by Roux is the analysis of development, not
directly into simple physico-chemical processes, but into more complex
organic processes dependent upon the fundamental properties of living
matter. The aim of _Entwicklungsmechanik_ is defined by Roux to be the
reduction of developmental events to the fewest and simplest
_Wirkungsweisen_, or causal processes.[483] Two classes of causal
processes may be distinguished, as "complex components" and "simple
components" of development. The latter are directly explicable by the
laws of physics and chemistry; the former, while in essence
physico-chemical, are yet so very complicated that they cannot at
present be reduced to physico-chemical terms. The ultimate aim of
_Entwicklungsmechanik_ is to reduce development to its "simple
components," but its main task at the present day and for many years to
come is the analysis of development into its "complex components."
These complex components must be accepted as having much of the validity
of physical and chemical laws. They are mysterious in the sense that
they cannot yet be explained mechanistically, but they are constant in
their action, and under the same conditions produce always the same
effect--hence they may be made the subject of strictly scientific study.
They represent biological generalisations, in their way of equal
validity with the generalisations of physics and chemistry.
The principal "complex components" which Roux recognises are somewhat as
follows:--First come the elementary cell-functions of assimilation and
dissimilation, growth, reproduction and heredity, movement and
self-division (as a special co-ordination of cell-movements). Then at a
somewhat higher level, self-differentiation, and the trophic reaction to
functional stimuli. Components of even greater complexity may also be
distinguished, as, for instance, the biogenetic law. The various
tropisms exhibited in development may be regarded as "directive" complex
components. There must be added, not as being itself a component, but
rather as a mode or peculiar property of all functioning, the
omnipresent faculty of self-regulation.
It will be noticed that Roux's "complex components" are simply the
general properties or functions of organised matter.
Expressing Roux's thought in another way, we might say that life can
only be defined functionally, _i.e._, by an enumeration of the "complex
components" or elementary functions which all living beings manifest,
even down to the very simplest. "Living beings," writes Roux, "can at
present be defined with any approach to completeness only functionally,
that is to say, through characterisation of their activities, for we
have an adequate acquaintance with their functions in a general way,
though our knowledge of particulars is by no means complete" (p. 105,
1905). Defined in the most general and abstract way, living things are
material objects which persist in spite of their metabolism, and, by
reason of their power of self-regulation, in spite also of the changes
of the environment. This is the "functional minimum-definition of life"
(pp. 106-7, 1905).
We may now go on to consider the relation of function to form throughout
the course of development. Roux distinguishes in all development two
periods, in the first of which the organ is formed prior to and
independent of its function, while in the second the differentiation and
growth of the organ are dependent on its functioning. Latterly (1906 and
1910) Roux has distinguished three periods, counting as the second the
transition period when form is partly self-determined, partly determined
by functioning. As this conception of Roux's is of the greatest
importance we shall follow it out in some detail.
The idea was first elaborated in the _Kampf der Theile_ (1881), where he
wrote:--"There must be distinguished in the life of all the parts two
periods, an embryonic in the broad sense, during which the parts
develop, differentiate and grow of themselves, and a period of completer
development, during which growth, and in many cases also the balance of
assimilation over dissimilation, can come about only under the influence
of stimuli" (p. 180). There is thus a period of self-differentiation in
which the organs are roughly formed in anticipation of functioning, and
a period of functional development in which the organs are perfected
through functioning and only through functioning. The two periods cannot
be sharply separated from one another, nor does the transition from the
one to the other occur at the same time in the different tissues and
organs.
The conception is more fully expressed in 1905 as follows:--"This
separation (of development into two periods) is intended only as a first
beginning. The first period I called the embryonic period [Greek: kat'
exochen] or the period of organ-rudiments. It includes the 'directly
inherited' structures, _i.e._, the structures which are directly
predetermined in the structure of the germ-plasm, as, for instance, the
first differentiation of the germ, segmentation, the formation of the
germ-layers and the organ-rudiments, as well as the next stage of
'further differentiation,' and of _independent_ growth and maintenance,
that is, of growth and maintenance which take place without the
functioning of the organs.
"This is accordingly the period of direct fashioning through the
activity of the formative mechanism implicit in the germ-plasm, also the
period of the self-conservation of the formed parts without active
functioning.
"The second period is the period of 'functional form-development.' It
includes the further differentiation and the maintenance in their
typical form of the organs laid down in the first period; and this is
brought about by the exercise of the specific functions of the organs.
This period adds the finishing touches to the finer functional
differentiation of the organs, and so brings to pass the 'finer
functional harmony' of all organs with the whole. The formative activity
displayed during this period depends upon the circumstance that the
functional stimulus, or rather the exercise by the organs of their
specific functions, is accompanied by a subsidiary formative activity,
which acts partly by producing new form and partly by maintaining that
which is already formed.... Between the two periods lies presumably a
transition period, an intermediary stage of varying duration in the
different organs, in which both classes of causes are concerned in the
further building-up of the already formed, those of the first period in
gradually decreasing measure, those of the second in an increasing
degree" (pp. 94-6, 1905).
In the first period the organ forms or determines the function, in the
second period the function forms the organ, or at least completes its
differentiation. It is characteristic that in the first period
functionally adapted structure appears in the complete absence of the
functional stimulus.
The explanation of the difference between the two periods is to be found
in the different evolutionary history of the characters formed during
each. First-period characters are _inherited_ characters, and taken
together constitute the historical basis of the organism's form and
activity; second-period characters are those of later acquirement which
have not yet become incorporated in the racial heritage.
Inherited characters appear in development in the absence of the
stimulus that originally called them forth; acquired characters are
those that have not yet freed themselves from this dependence upon the
functional stimulus. First-period characters were originally, like
second-period characters, entirely dependent for their development upon
the functional stimuli in response to which they arose, and only
gradually in the course of generations did they gain that independence
of the functional stimulus which stamps them as true inherited
characters. Speaking of the formative stimuli which are active in
second-period development, Roux writes:--"These stimuli can also produce
new structure, which if it is constantly formed throughout many
generations finally becomes hereditary, _i.e._, develops in the
descendants in the absence of the stimuli, becomes in our sense
embryonic" (p. 180, 1881). Again, "form-characteristics which were
originally acquired in post-embryonic life through functional adaptation
may be developed in the embryo without the functional stimulus, and may
in later development become more or less completely differentiated, and
retain this differentiation without functional activity or with a
minimum of it. But in the continued absence of functional activity they
become atrophied ... and in the end disappear" (p. 201, 1881).
This conception of the nature of hereditary transmission is an important
one, and constitutes the first big step towards a real understanding of
the historical element in organic form and activity. It supplies a
practical criterion for the distinguishing of "heritage" characters from
acquired characters, of palingenetic from cenogenetic--a criterion which
descriptive morphology was unable to find.[484] The introduction of a
functional moment into the concept of heredity was a methodological
advance of the first importance, for it linked up in an understandable
way the problems of embryology, and indirectly of all morphology, with
the problem of hereditary transmission, and gave form and substance to
the conception of the organism as an historical being.
It is this element in Roux's theories that puts them so far in advance
of those of Weismann. Weismann did not really tackle the big problem of
the relation of form to function, and he left no place in his mechanical
system of preformation for functional or second-period development; he
conceived all development to be in Roux's sense embryonic, and due to
the automatic unpacking of a complex germinal organisation. Roux himself
was to a certain extent a preformationist, for the development of his
first-period characters is conditioned by the inherited organisation of
the germ-plasm, and is purely automatic. It was indeed his experiments
on the frog's egg (1888) that supplied some of the strongest evidence in
favour of the mosaic theory of development. The number of _Anlagen_
which he postulates in the germ is however small, and the germ-plasm in
his conception of it has a relatively simple structure (p. 103, 1905).
The transmission of acquired characters forms, of course, an integral
part of Roux's conception of heredity and development, for without this
transmission second-stage characters could not be transformed into
first-stage characters. He discusses this difficult question at some
length in the _Kampf der Theile_, coming to the conclusion that such
transmission takes place in small degree and gradually, and that many
generations are required before a new character can become hereditary.
He thinks that acquired characters are probably transmitted at the
chemical level. It is conceivable that acquired form-changes are
dependent on chemical changes, or are correlative with such, and that,
since the germ-cells stand in close metabolic relations with the soma,
these chemical changes may soak through to the germ-cells and so modify
them that a predisposition will appear in the descendants towards
similar form-changes.[485] From this point of view the problem of
transmission might be merged in the broader problem of the production of
form through chemical processes--the central problem of all development.
Inherited characters develop by an automatic process of
self-differentiation, and the separate parts of the embryo show during
this first period a surprising functional independence of one another.
But this state of things changes progressively as the second period is
reached, until finally all form-production and maintenance and all
correlation depend upon functioning. It is in the first period of
automatic development through internal "determining" factors that the
"developmental" functions in the strict sense, _e.g._ automatic growth,
division and self-differentiation, are most clearly shown. In the second
or "functional" period the formative influence of function upon
structure comes into play, and development becomes largely a matter of
"functional adaptation" to functional requirements.
All structure, according to Roux, is either functional or
non-functional. The former includes all structure that is adapted to
subserve some function. "Such 'functional structures' are, for example,
the composition of striated muscle fibres out of fibrillae and these out
of muscle-prisms, or again the length and thickness of the muscles, the
static structure of the bones, the composition of the stomach and the
blood-vessels out of longitudinal and circular fibres, the external
shape of the vertebral centra and of the cuneiform bones of the foot"
(p. 73, 1910). Indeed, as Cuvier had already pointed out, practically
every organ in the body shows a functional structure which is accurately
and minutely adjusted to the function it is intended to perform. Thus,
to take some further examples, the arteries are admirably adapted as
regards size of lumen, elasticity of wall, direction of branching, to
conduct the blood to all parts of the body with the least possible waste
of the propelling power through frictional resistance. So, too, the
spongy substance of the long bones is arranged in lamellae which take the
direction of the principal stresses and strains which fall upon the
bones in action.
Functional structure may be formed either in the first or in the second
period of development, may be either inherited or acquired, but it
reaches its full differentiation only in the second period, _i.e._,
under the influence of functioning. Practically speaking, functional
structure is directly dependent for its full development and for its
continued conservation upon the exercise of the particular function
which it serves. In the second period, but not in the first, increased
use leads to hypertrophy of the functional structure, disuse to atrophy.
From functional structure is to be distinguished nonfunctional
structure, which has no relation to the bodily functions--is neither
adapted to perform any of these, nor has arisen as a by-product of
functional activity. "To this category belong, for example, among
typical structures, the triangular form of the cross-section of the
tibia, the dolicocephalic or brachycephalic shape of the skull, most of
the external characters distinguishing genera and species, many of the
external features of the embryo which change in the course of
development, besides most of the abnormal forms shown by monstrosities,
tumours, etc." (p. 74, 1910). Non-functional structure is not affected
by functional adaptation, and may accordingly be left out of
consideration here.
Now the influence of functioning upon the form and structure of an organ
is twofold. There is first the immediate change brought about by the
very act of functioning--for example, the shortening and thickening of
skeletal muscles when they act. This is a purely temporary change, for
the organ at once returns to its normal quiescent state as soon as it
ceases to function. Such temporary functional change, brought about in
the moment of functioning, is usually dependent for its initiation upon
some neuro-muscular mechanism, though it may be elicited also by a
chemical stimulus. It is thus always a phenomenon of "behaviour." "From
such temporary changes are sharply to be distinguished all permanent
alterations which first appear in perceptible fashion through
oft-repeated or long-continued, enhanced functional activity. These
produce a new and lasting internal equilibrium of the organ, consisting
in an insertion of new molecules or a rearrangement of old. For this
reason they outlast the periods of functional form-change, or, if as in
the case of the muscles they themselves alter during functional
activity, they regain their state when the organ ceases to function" (p.
72, 1910). "Oft-repeated exercise or heightened exercise of the specific
functions, or repeated action of the functional stimuli which determine
them, produces, as we have said before, true form-changes as a
by-product. These are of two kinds. In so far as these form-changes
facilitate the repetition of the specific functions, I have called them
_functional adaptations_.... Such as do not improve the functioning of
the organ are indeed by-products of functioning, but without adaptive
character; they do not belong to the class of functional adaptations at
all" (p. 75, 1910).
We may now enquire in what way functional adaptations can arise as
by-products of functioning.
It is clear that natural selection in the sense of individual or
"personal" selection cannot adequately explain the origin of functional
structure and the functional harmony of structure, for thousands of
cells would have to vary together in a purposive way before any real
advantage could be gained in the struggle for existence, and it is in
the highest degree unlikely that this should come about by chance
variation.[486] The development of purposive internal structure is only to
be explained by the properties of the tissues concerned.
In illustration and proof of the statement that functional adaptation is
due to the properties of the tissues we may adduce the development and
regulation of the blood-vascular system, which has been thoroughly
studied from this point of view by Roux and Oppel (1910).
It appears that only the very first rudiments of the vascular system are
laid down in the short first period of automatic non-functional
development. All the subsequent growth and differentiation of the
blood-vessels falls into the second period, and is due wholly or in
great part to direct functional adaptation to the requirements of the
tissues. Thus from the rudiments formed in the first period there sprout
out the definitive vessels in direct adaptation to the food-consumption
of the tissues they are to supply. The size, direction and intimate
structure of these vessels are accurately adjusted to the part they play
in the economy of the whole, and this adjustment is brought about in
virtue of the peculiar properties or reaction-capabilities of the
different tissues of which the blood-vessels are composed.
The properties which Roux finds himself compelled to postulate in the
vascular tissues, after a thorough-going analysis of the different kinds
of functional adaptation shown by the blood-vessels, are summarised by
him as follows:--
"(1) The faculty--depending on a direct sensibility possessed by the
endothelium and perhaps also by the other layers of the intima--of
yielding to the impact of the blood, so far as the external relations of
the vessel permit. In this way the wall adapts itself to the
haemodynamically conditioned 'natural' shape of the blood-stream, and
reaches this shape as nearly as possible." Through this faculty of the
lining tissue of the blood-vessels, the size of the lumen and the
direction of branching are so regulated as to oppose the least possible
resistance to the flow of the blood.
"(2) The faculty possessed by the endothelium of the capillaries of each
organ of adapting itself qualitatively to the particular metabolism of
the organ." This adaptedness of the capillaries is, however, more
usually an inherited state, _i.e._, brought about in the first period of
development.
"(3) The faculty possessed by the capillary walls of being stimulated to
sprout out and branch by increased functioning, _i.e._, by increased
diffusion, and their power to exhibit a chemically conditioned
cytotropism, which causes the sprouts to find one another and unite. A
similar process can be directly observed in isolated segmentation-cells,
which tend to unite in consequence of a power of mutual attraction.
"(4) The faculty of developing normal arterial walls in response to
strong intermittent pressure, and normal venous walls in response to
continuous lesser pressure." It has been shown, for instance, by Fischer
and Schmieden that in dogs a section of vein transplanted into an artery
takes on an arterial structure, at least as regards the circular
musculature, which doubles in thickness.
"(5) The power to regulate the normal[487] length of the arteries and
veins, in adaptation to the growth of the surrounding tissues, in such a
way that the stretching action of the blood-stream brings the vessel to
its proper functional length.
"(6) The power to form, in response to slight increases in longitudinal
tension, new structural parts which take their place alongside the
existing longitudinal fibres.
"(7) The power to regulate the width of the circular musculature
according to the degree of food-consumption by the tissues, in response
to nerve impulses initiated in these tissues.
"(8) The power possessed by the circular musculature of responding to
such continuous functional widening, by the formation of new structural
parts in the circular musculature, and so of widening the vessel
permanently or by this new formation of muscular fibres thickening the
circular musculature.
"(9) The faculty of being stimulated by increased blood-pressure to
produce the same structural changes as mentioned in par. 8, though here
the response is otherwise conditioned" (pp. 126-7, 1910).
It is by virtue of the tissue-properties detailed above that the complex
functional adaptations of the blood-vessels come about.
The development of the vascular system is no mere automatic and
mechanical production of form, apart from and independent of
functioning; it implies a living and co-ordinated activity of the
tissues and organs concerned, a power of active response to foreseen and
unforeseen contingencies. Form is then not something fixed and
congealed--it is the ever-changing manifestation of functional activity.
"Since most of the structure and form of the blood-vessels arises in
direct adaptation to function, the vessels of adult men and animals are
no fixed structures, which, once formed, retain their form and
structural build unchanged throughout life; on the contrary, they
require even for their continued existence the stimulus of functional
activity.... The fully formed blood-vessels are no static structures,
such as they appear to be according to the teaching of normal histology,
and such as they have long been taken to be. Observation and description
of normal development never shows us anything but the visible side of
organic happenings, the _products_ of activity, and leaves us ignorant
of the real processes of form-development and form-conservation, and of
their causes" (p. 125, 1910).
The real thing in organisation is not form but activity. It is in this
return to the Cuvierian or functional attitude to the problems of form
that we hold Roux's greatest service to biology to consist. The
attitude, however, seems to smack of vitalism, and Roux, as we have
seen, is no vitalist. He holds that the marvellous and apparently
purposive tissue-qualities which underlie all processes of functional
adaptation have arisen "naturally," in the course of evolution, by the
action of natural selection upon the various properties, useful and
useless, which appeared fortuitously in the primary living organisms. He
is, moreover, deeply imbued with the materialistic philosophy of his
youth, and it is indeed one of the chief characteristics of his system
that he states the fundamental properties or qualities of life in terms
of metabolism. A vital quality is for Roux a special process or mode of
assimilation. The faculty of "morphological assimilation" whereby form
is imposed upon formless chemical processes is the ultimate term of
Roux's analysis--"the most general, most essential, and most
characteristic formative activity of life" (p. 631, 1902).
We have now to consider very briefly the early results achieved by
Roux's fellow-workers in the field of causal morphology. As D. Barfurth
points out,[488] the years 1880-90 saw a general awakening of interest in
experimental morphology, and it is hard to say whether Roux's work was
cause or consequence. "There fall into this period," writes Barfurth,
"the experimental investigations by Born and Pflueger on the sexual
difference in frogs (1881), by Pflueger on the parthenogenetic
segmentation of Amphibian ova, on crossing among the Amphibia, and on
other important subjects (1882). In the following year (1883) appeared
two papers of fundamental importance, by E. Pflueger and W. Roux: Pflueger
publishing his researches on 'the influence of gravity on
cell-division,' Roux his experimental investigations on 'the time of the
determination of the chief planes in the frog-embryo.'... In the same
year appeared A. Rauber's experimental studies 'on the influence of
temperature, atmospheric pressure, and various substances on the
development of animal ova,' which have brought many similar works in
their train. The following year (1884) saw a lively controversy on
Pflueger's gravity-experiments with animal eggs, in which took part
Pflueger, Born, Roux, O. Hertwig and others, and in this year appeared
work by Roux dealing with the experimental study of development, and in
particular giving the results of the first definitely localised
pricking-experiments on the frog's egg (in the _Schles. Gesell. f.
vaterl. Kultur_, 15th Feb. 1884), also the important researches of M.
Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani)
on Protozoa, and other experimental work" (pp. xi.-xii.).
In 1888 appeared a famous paper by W. Roux,[489] in which he described how
he had succeeded in killing by means of a hot needle one of the two
first blastomeres of the frog's egg, and how a half-embryo had developed
from the uninjured cell. Some years before[490] he had enunciated, at
about the same time as Weismann, the view that development was brought
about by a qualitative division of the germ-plasm contained in the
nucleus, and that the complicated process of karyokinetic or mitotic
division of the nucleus was essentially adapted to this end. He
conceived that development proceeded by a mosaic-like distribution of
potencies to the segmentation-cells, that, for instance, the first
segmentation furrow separated off the material and potencies for the
right half of the embryo from those for the left half. He had tried to
show experimentally that the first furrow in the frog's egg coincided
with the sagittal plane of the embryo,[491] and his later success in
obtaining a half-embryo from one of the first two blastomeres seemed to
establish the "mosaic theory" conclusively.
Roux's needle-experiment aroused much interest, especially as Weismann's
theory of heredity was then being keenly discussed. Chabry had published
in 1887 some interesting results on the Ascidian egg,[492] which strongly
supported the Roux-Weismann theory. Considerable astonishment was
therefore caused by Driesch's announcement in 1891[493] that he had
obtained complete larvae from single blastomeres of the sea-urchin's egg
isolated at the two-celled stage. He followed this up in the next
year[493] by showing that whole embryos could be produced from one or more
blastomeres isolated at the four-cell stage. Similar or even more
striking results were obtained by E. B. Wilson on _Amphioxus_,[494] and
Zoja on medusae.[495] Driesch succeeded also in disturbing the normal
course and order of segmentation by compressing the eggs of the
sea-urchin between glass plates, and yet obtained normal embryos.
Similar pressure-experiments were carried out on the frog by O.
Hertwig,[496] and on _Nereis_ by E. B. Wilson,[497] with analogous results.
In 1895 O. Schultze[498] showed that if the frog's egg is held between two
plates and inverted at the two-celled stage there are formed two embryos
instead of one. In the same year T. H. Morgan[499] repeated Roux's
fundamental experiment of destroying one of the two blastomeres, but
inverted the egg immediately after the operation--a whole embryo of half
size resulted. A year or two later Herlitzka[500] found that if the first
two blastomeres of the newt's egg were separated by constriction, two
normal embryos of rather more than half normal size were formed.
The main result of the first few years' work on the development of
isolated blastomeres was to show that the mosaic theory was not strictly
true, and that the hypothesis of a qualitative division of the nucleus
was on the whole negatived by the facts.
Evidence soon accumulated that the cytoplasm of the egg stood for much
in the differentiation of the embryo. A number of years previously Chun
had made the discovery that single blastomeres of the Ctenophore egg,
isolated at the two-celled stage, gave half-embryos. This was in the
main confirmed by Driesch and Morgan in 1896,[501] and they made the
further interesting discovery that the same defective larvae could be
obtained by removing from the unsegmented egg a large amount of
cytoplasm. Conclusive proof of the importance of the cytoplasm was
obtained soon after by Crampton,[502] who removed the anucleate
"yolk-lobe" from the egg of the mollusc _Ilyanassa_ at the two-celled
stage, and obtained larvae which lacked a mesoblast. This result was
brilliantly confirmed and extended some years later by E. B. Wilson,[503]
working on the egg of _Dentalium_. He found that if the similar
anucleate "polar lobe" of this form is removed at the two-celled stage,
deficient larvae are formed, in which the post-trochal region and the
apical organ are absent. He further showed that in the unsegmented but
mature egg prelocalised cytoplasmic regions can be distinguished, which
later become separated from one another through the segmentation of the
egg. The segmentation-cells into which these cytoplasmic substances are
thus segregated show a marked specificity of development, giving rise,
even when isolated, to definite organs of the embryo. Wilson concluded
that the cytoplasm of the egg contains a number of specific
organ-forming stuffs, which have a definite topographical arrangement in
the egg. Development is thus due in part to a qualitative division not
of the nucleus but of the cytoplasm. Corroborative evidence of the
existence of cytoplasmic organ-forming stuffs has been supplied for
several other species, _e.g._, _Patella_ (Wilson), _Cynthia_ (Conklin),
_Cerebratulus_ (Zeleny), and _Echinus_ (Boveri).
It is interesting to recall that so long ago as 1874 W. His[504] put
forward the theory that there exist in the blastoderm and even in the
egg prelocalised areas, which contain the formative material for each
organ of the embryo, and from which the embryo is developed by a simple
process of unequal growth.
The experimental study of form was prosecuted in many other directions
besides that of experimental embryology. The study of regeneration and
of regulatory processes attracted many workers, among whom may be
mentioned T. H. Morgan, C. M. Child, and H. Driesch. In an interesting
series of papers C. Herbst applied the principles of the physiology of
stimulus to the interpretation of development.[505] The formative power of
function was studied in Germany by Roux and his pupils, Fuld, O. Levy,
Schepelmann and others, particularly by E. Babak. In France, F. Houssay
inaugurated[506] an important series of memoirs by himself and his pupils
on "dynamical morphology," the most important memoir being his own
valuable discussion of the functional significance of form in fishes.[507]
The principles of his dynamical morphology were first laid down in his
book _La Forme et la Vie_ (1900).
The famous experiments of Loeb, Delage and others on artificial
parthenogenesis may also be mentioned, though their connection with
morphology is somewhat remote.
The period was characterised also by the lively discussion of first
principles, in which Driesch took a leading part. Materialistic methods
of interpretation were upheld by perhaps the majority of biologists, but
vitalism found powerful support.
[464] See Carus's remark, referred to on p. 194, above.
[465] Roux, _Die Entwicklungsmechanik_, p. 26, Leipzig,
1905.
[466] T. H. Morgan, _Regeneration_, p. 1, New York and
London, 1901.
[467] _Recherches sur la production artificielle des
Monstruosites_, Paris, 1877, and many later papers.
[468] _Unsere Koerperform und das physiologische Problem
ihrer Entstehung_, Leipzig, 1874.
[469] J. W. Jenkinson, _Experimental Embryology_, p. 3,
Oxford, 1909.
[470] "Ueber die Verzweigungen der Blutgefaesse des
Menschen," _Jen. Zeit_., xii., 1878.
[471] "Ueber die Bedeutung der Ablenkung des
Arterienstammes bei der Astabgabe," _Jen. Zeit_., xiii.,
1879.
[472] "Beitraege zur Morphologie der funktionellen
Anpassung. I. Struktur eines hochdifferenzierten
bindgewebigen Organes (der Schwanzflosse des Delphin),"
_Arch. Anat. Physiol._ (_Anat. Abt._) for 1883. II.
"Ueber die Selbstregulation der 'morphologischen' Laenge
der Skeletmuskeln des Menschen," _Jen. Zeit._, xvi.,
1883. III. "Beschreibung ... einer
Kniegelenkeknochenankylose," _Arch. Anat. Physiol._
(_Anat. Abt._) for 1885.
[473] In 1869 and 1877 respectively (Roux, p. 53, 1905).
[474] _Ueber die Zeit. der Bestimmung der Hauptrichtungen
des Froschembryo_, Leipzig, 1883.
[475] "Ueber den Einfluss der Schwerkraft auf die Teilung
der Zellen," Pflueger's _Archiv_, xxxi., 1883. Also
subsequent papers in same journal.
[476] For an account of the classical experiments on the
frog's egg, see T. H. Morgan, _The Development of the
Frog's Egg_, New York, 1897.
[477] In a series of "Beitraege zur Entwicklungsmechanik
des Embryo," published in various journals from 1884 to
1891, all dealing with the frog's egg. Also in many
papers in the _Archiv f. Entw. mech._, from 1895
onwards.
[478] _Die Entwicklungsmechanik der Organismen, eine
anatomische Wissenschaft der Zukunft_, Wien, 1890.
[479] The first volume contains the important _Einleitung_
or general Introduction.
[480] _Gesammelte Abhandlungen ueber Entwicklungsmechanik
der Organismen_, 2 vols., Leipzig, 1895.
[481] "Fuer unser Programm und seine Verwirklichung,"
_A.E.M._, v., pp. 1-80 and 219-342, 1897. "Ueber die
Selbstregulation der Lebewesen," _A.E.M._, xiii., pp.
610-5, 1902. "Die Entwicklungsmechanik, ein neuer Zweig
der biologischen Wissenschaft," Heft I. of the _Vortraege
u. Aufsaetze ueber Entwicklungsmechanik der Organismen_,
Leipzig, 1905. Oppel and Roux, "Ueber die gestaltliche
Anpassung der Blutgefaesse," Heft x., of the _Vortraege u.
Aufsaetze_, Leipzig, 1910.
[482] "Ueber d. funkt. Anpassung des Muskelmagens der
Gans," _A.E.M._, xxi., pp. 461-99, 1906.
[483] The exact quantitative formulation of a
_Wirkungsweise_ constitutes a law. The word itself is
perhaps most conveniently rendered as "causal process."
[484] M. Fuerbringer, perhaps under the influence of Roux,
emphasised the importance, from a morphological point of
view, of studying post-embryonic (functional)
development, _Unters. z. Morph. u. Syst. der Voegel_,
ii., Amsterdam, p. 925, 1888.
[485] See, for the development of this idea, Oppel, in
Roux-Oppel, 1910.
[486] _Cf._ the controversy between Herbert Spencer and
Weismann on the subject of "coadaptation" in the
_Contemporary Review_ for 1893 and 1894. See also
Weismann's paper in _Darwin and Modern Science_,
Cambridge, 1909.
[487] That is, the length they take up when separated from
the body.
[488] "Wilhelm Roux zum 60. Geburtstage," _Arch. f.
Entw.-Mech._, xxx. _Festschrift fuer Prof. Roux_, Pt. i,
1910.
[489] Virchow's _Archiv_, cxiv., 1888. First announced in
Sept. 1887.
[490] _Ueber die Bedeutung der Kernteilungsfiguren_,
Leipzig, 1883.
[491] _Bresl. aertz. Zeitschr._, 1885.
[492] _Journ. de l'Anat. et de la Physiologie_, xxiii.,
1887.
[493] _Zeits. f. wiss. Zool._, liii., 1891 and 1892.
[494] _Journ. Morph._, viii., 1893.
[495] _Arch. f. Ent.-Mech._, i., 1895; ii., 1896.
[496] _Arch. f. mikr. Anat._, xliii., 1893.
[497] _Arch. f. Ent.-Mech._, iii., 1896.
[498] _Arch. f. Ent.-Mech._, i., 1895.
[499] _Anat. Anz._, x., 1895.
[500] _Arch. f. Ent.-Mech._, iv. 1897.
[501] _Arch. f. Ent.-Mech._, ii., 1896.
[502] _Arch. f. Ent.-Mech._, iii., 1896.
[503] _Journ. exper. Zool._, i., 1904.
[504] _Unsere Koerperform_, p. 19, Leipzig, 1874.
[505] _Biolog. Centrlbl._, xiv., 1894, xv., 1895.
_Formative Reize in der thierischen Ontogenese_,
Leipzig, 1901.
[506] "La Morphologie dynamique," No. i. of the
_Collection de Morphologie dynamique_, Paris, 1911.
[507] "Forme, Puissance et Stabilite des Poissons," No.
iv. of the _Collection_, Paris, 1912.