E, Diagrammatic representation of the sensory epithelium of retina
(rod and cone layer). (After Schwalbe.)]
(c) _Specialization for the Reception and Conduction of Stimuli._--One
of the most striking of the fundamental attributes of living protoplasm
is its "irritability," that is to say, its power of responding to
external impressions, "stimuli," by movement, which, both in kind and
intensity, is wholly independent of the amount of energy expended by the
stimulus. The stimulus conveyed by the nerve fibre to the muscle is out
of all proportion to the amount of work it may cause the muscle to do.
Although protoplasmic irritability is thus incapable of a simple
mechanical explanation, science has rejected the assumption of a special
"vital force," and interprets protoplasmic response as being a long
series of chemico-physical changes,[13] initiated, but only initiated,
by the original stimulus; the latter thus standing in the same relation
to the response it produces as the pull on the trigger to the propulsion
of the rifle bullet. The function of receiving stimuli from the outer
world, originally possessed to a greater or less extent by all cells,
has, in the Metazoa, been relegated to one class of cells, the sensory
cells[14] (fig. 5, D and E). Another class of cells--the "ganglion
cells" or "neurones" (fig. 5, A and B), are concerned with the
conduction of the stimuli so received. The contractile elements in the
Metazoa are thus dependent for their stimuli on the nervous
elements--the sensory cells and neurones.
_Origin of Cells._--In the preceding sections we have considered the
structure of the cell in relation to the fundamental attributes of
cell-metabolism, irritability, and movement. We have now to consider
the cell in relation to yet another vital attribute, that of
reproduction. Just as we now know that the phenomena of assimilation,
respiration, excretion, response, movement and so forth, characteristic
of living things, are but the co-ordinated expressions of the
corresponding activities of the constituent cells, so we now know that
the reproduction of the organism is, in its ultimate analysis, a
cell-process. Our knowledge of the essential fact that cells only arise
by the division of pre-existing cells, now a fundamental axiom of
biology, and of the details of this process, have been acquired during
recent years by the strenuous efforts of numerous workers.[15] Matthias
Jakob Schleiden (1838) supposed that in plants the new cell arose from
the parent cell by a sort of "crystallizing" process from the cell fluid
or "cytoblastema"; the nucleolus appearing first, then the nucleus, and
finally the cell-body. Theodor Schwann (1839) extended Schleiden's
theory to animal tissues, with this yet greater error, that new cells
might arise, not only within the mother cell as Schleiden had supposed,
but also in the intercellular substance so common in animal tissues (to
which he also gave the term "cytoblastema"). By 1846, however, the
botanists, thanks mainly to the efforts of Hugo von Mohl and Nageli,
recognized as a general law that cells only arise by the division of a
pre-existing cell. But it was long before the universal application of
this law was recognized by zoologists; the delay being largely due to
pathological phenomena. The work of Kolliker (1844-1845), Karl Bogislaus
Reichert (1841-1847), and Remak (1852-1855), however, finally enabled
Virchow in 1858 to maintain the law of the genetic continuity of cells
in the since famous aphorism _omnis cellula e cellula_. At this time,
however, nothing was known of the details of cell-division,--one school
(Reichert, L. Auerbach, and the majority of the botanists) maintaining
that the nucleus disappeared prior to cell-division, the other school
(von Baer, Remak, Leydig, Haeckel, &c.) maintaining that it took a
leading part in the process. It is not until the appearance of Anton
Schneider's work in 1873, followed by those of Fol, Auerbach,
Strasburger and many others, that we begin to gain an insight into the
process. In 1882 W. Flemming was able to extend Virchow's aphorism to
the nucleus also: _omnis nucleus e nucleo_.
_Outline of Cell-division._--There are two very distinct methods of
cell-division. The more general and also more complicated method is
accompanied by the formation of a complex fibrillar mechanism, and was
on this account termed "mitosis" ([Greek: mitos], a thread) by W.
Flemming (1882), and "karyokinesis" ([Greek: karyon], nut, nucleus, and
[Greek: kinesis], change, movement) by W. Schleicher (1878). The other
method, "amitosis," or direct division, is unaccompanied by any visible
mechanism and is of relatively exceptional occurrence. In the more usual
method of cell-division, or "mitosis," we can distinguish two distinct
but parallel processes, the one undergone by the chromatin and resulting
in the "chromatic figure," the other usually only concerning the
cytoplasm and resulting in the "achromatic figure."[16]
[Illustration: a, b and c from Prof. E. B. Wilson's _The Cell in
Development and Inheritance_, by permission of the author and the
Macmillan Co., New York; d from A. Gurwitsch, _Morphologie u. Biologie
der Zelle_, by permission of Gustav Fischer.
FIG. 6.--Diagram of Nuclear Division. a, Spireme stage; b, Spindle
formed; c, Spindle complete; equatorial plate formed; d, Division
completed.]
We will consider the chromatin changes first. The chromatin granules
lose their scattered arrangement on the nuclear reticulum, and become
instead arranged in a linear series to form a coiled and deeply staining
"spireme thread"[17] (fig. 6, a). As the thread contracts, its granular
origin becomes less evident, and at the same time the coils become fewer
in number; the "close" spireme of earlier stages becomes the "loose"
spireme of later stages. As the spireme thread contracts, it segments
into a number of short, and usually U-shaped, segments--the
"chromosomes" (Waldeyer, 1888). The number of these chromosomes is
always constant for the cells of any given species of plant or animal,
but varies greatly in number in different species. Thus in the
parasitic worm _Ascaris megalocephala_, var. _univalens_, there are only
two. In the crustacean _Artemia_ Bauer found 168, while in the amphibian
_Salamandra maculata_, as also in the lily, the number is 24. While
these changes have been proceeding in the nucleus, changes in the
cytoplasm have resulted in the formation of the achromatic figure. These
cytoplasmic changes are initiated by the division into two of a minute
body, the "centrosome," originally discovered by P. J. van Beneden in
1883,[18] and usually lying not far from the nucleus (fig. 6, a). The
daughter centrosomes separate from one another, travelling to opposite
poles of the nucleus. At the same time radiations extend out into the
cytoplasm from the centrosomes, and, as the nuclear membrane disappears,
invade the nuclear area (fig. 7, a). Some of the fibrillae in the latter
region become attached to the chromosomes and are termed "mantle
fibres"; others become continuous from one centrosome to the other and
constitute the "spindle fibres." The remaining radiations at the two
poles of the spindle are the "astral rays." (The details of the
formation of the achromatic figure vary considerably, some indication of
this is given in the next section in connexion with the question of the
origin of the mitotic mechanism.) The chromosomes now arrange themselves
in the "equatorial plate" of the spindle and each splits longitudinally
into two[19] (fig. 6, b and c). The sister chromosomes now pass to
opposite poles of the spindle (fig. 6, d), and there, returning to the
"resting" condition, constitute the daughter nuclei. Division of the
cell follows, usually, in animals, by simple constriction. Both Theodor
Boveri and van Beneden, in their papers of 1887, regarded the centrosome
as initiating, not only the division of the cell-body but that of the
chromatin also; Beneden even suggested that the pull of the mantle
fibres caused the division of the chromatin in the equatorial plate. W.
Pfitzner in 1882 was the first to show that the splitting of the
chromosomes in the equatorial plate was only the reappearance of a split
in the spireme thread and was due to a corresponding division into two
of each of the chromatin granules. In the spermatogenic cells of
_Ascaris_, A. Brauer has shown that the chromatin granules divide while
still scattered over the nuclear reticulum and before either the
formation of a spireme thread or the division of the centrosome. In many
other cases the reverse of this condition occurs, the centrosome
dividing long before there is any indication of division in the nucleus
(e.g. salamander spermatogenic cells, Meves, &c.). We must therefore,
with Boveri and Brauer, regard the division of the chromatin in mitosis
as a distinct reproductive act on the part of the chromatin granules,
the chromosomes being merely aggregates (temporary or permanent, _vide
infra_) of these self-propagating units.
For convenience of description it is usual to recognize four periods in
mitosis: (i.) Prophase, (ii.) Metaphase, (iii.) Anaphase, and (iv.)
Telophase (Strasburger, 1884). The prophase covers all changes up to the
completion of the mitotic figure. The metaphase is the parting of the
sister chromosomes in the equatorial plate; their passage to opposite
poles of the spindle constitutes the anaphase; and their reconstruction
to form the resting daughter nuclei, the telophase.
[Illustration: FIG. 7.--Centrosomes.
From Prof. E. B. Wilson's _The Cell in Development and Inheritance_, by
permission of the author and of The Macmillan Co., New York.
a, Leucocyte from a Salamander, showing permanent aster and
centrosome.
From A. Gurwitsch, _Morphologie u. Biologie der Zelle_, by permission of
Gustav Fischer.
b, Sperm-mother cell of _Salamandra maculata_, showing Hermann's
"central spindle."]
_The Achromatic Figure._--The mode of origin of the achromatic figure
varies greatly. In some cases a distinct and continuous spindle, the
"central spindle" of F. Hermann, is visible from the very first
separation of the daughter centrosomes (e.g. salamander spermatogenic
cell)[20] (fig. 7, b). In other cases the rays only invade the nuclear
area and become continuous in the equatorial plane after the centrosomes
have assumed their definitive positions at the two poles of the nucleus,
and may even appear to indent the disappearing nuclear membrane as they
invade the nuclear area.[21] In the salamander testis cell (fig. 7, b),
and in many other cases, the whole of the achromatic figure is obviously
of cytoplasmic origin. In many cases, however, it equally obviously
arises within the nucleus,[22] while in yet other cases[23] the spindle
fibres are of mixed origin. The question, therefore, of the cytoplasmic
or nuclear origin of the achromatic figure, at one time regarded as of
considerable importance, is wholly immaterial. Various elaborate
theories have been propounded to explain the mechanism of the mitotic
figure. H. Fol (1873) regarded the centrosomes as centres of attractive
forces, and compared the mitotic figure to the lines of force in the
magnetic field, a comparison made by numerous subsequent workers. E.
Klein's hypotheses of two opposing systems of contractile fibrillae,
elaborated by van Beneden (1883, 1887) and accepted by Boveri (1888),
was still further extended by R. Heidenhain in relation to the
leucocytes of the salamander, in which there is a permanent centrosome
and astral rays to which the contractile movements of the cell appear to
be due[24] (fig. 7, a). Hermann on the other hand confined the
contractility to the astral and mantle fibres; while L. Druner regarded
the spindle as exerting a pushing force, for not only do the interzonal
spindle fibres elongate during the anaphase, but they were often at this
period contorted, while on the other hand astral rays may be entirely
absent (e.g. Infusoria), and in some cases the spindle pole may be
caused to project at the surface of the cell. The futility of these
attempted mechanical explanations of mitosis is sufficiently clearly
shown, not only by the contradictory nature of the explanations
themselves, but by the fact that, in amitosis, nuclear and cytoplasmic
division occur without any fibrillar mechanism whatever.
_Centrosome._[25]--This minute body was first detected at the spindle
poles by Flemming in 1875, and independently by P. J. van Beneden in
1876. The important part played by the centrosome in fertilization,[26]
first described by van Beneden and Theodor Boveri in their papers of
1887-1888, together with the behaviour of this structure in mitosis, led
these authors to regard the centrosome not only as the dynamic centre of
the cell but as a permanent cell-organ, which, like the nucleus, passed
by division from one cell-generation to the next. This conclusion
appeared to receive considerable support from the recognition of the
centrosome in various kinds of resting cells,[27] and especially from
the relation this structure frequently shows to the locomotor apparatus
of the cell (e.g. its position in the centre of the radiating fibrillae
in the contractile lymph and pigment cells, and its relation to the
vibratile flagellum in spermatozoa and some protozoa, e.g.
Trypanosoma).[28] In almost all cases the centrosome of the resting
cell, when this can be detected, lies in the cytoplasm, and is often
already divided in preparation for the next mitotic division (e.g.
spermatogenic cells of the salamander; Meves). In some cases, however,
it resides in, or arises from, the nucleus (Brauer; spermatogenesis of
_Ascaris_, var. _univalens_). This indifferent nuclear or cytoplasmic
position for the centrosome is paralleled by the attraction sphere or
homologue of the centrosome in many Protozoa. Thus in many forms, e.g.
_Euglena_ (Keuten), it lies within the nucleus, while in other forms,
e.g. _Noctiluca_ (Ishikawa, 1894, 1898; Calkins, 1898) and _Paramoeba_
(F. Schaudinn, 1896), it lies in the cytoplasm, while in _Tetramitus_ it
coexists with a "distributed" nucleus. In the Heliozoa conditions are
exceptionally interesting; not only is the centrosome--here resembling
in appearance that of the higher forms--permanently visible and
extranuclear, lying at the centre of the radiations characteristic of
these forms, but there is the strongest possible evidence for its
formation _de novo_. For Schaudinn has shown in _Acanthocystis_ that, in
the formation of the swarm spores, the nucleus divides amitotically, the
centrosome remaining visible and unchanged at the centre of the
radiating processes. Yet a centrosome appears later in the nucleus of
the swarm spores and migrates into the cytoplasm. The experiments of T.
H. Morgan and E. B. Wilson, in which numerous centrosomes and asters
("cytasters") are caused to appear in unfertilized sea-urchin eggs by a
brief immersion in a 13% solution of magnesium chloride in
sea-water,[29] as also the possibility in many cases that even in normal
fertilization the cleavage centrosomes may arise _de novo_,[30] make it
no longer possible to regard the centrosome as a permanent
cell-structure.
_Significance of Mitosis._--Whatever may be the nature of the
chemico-physical changes occurring during cell-division, of which the
achromatic spindle and astral rays are the visible expression, it is
certain that the whole of this complicated process has for its function,
not the division of the chromatin, for that has already occurred on the
spireme thread or even earlier, but the distribution of the divided
chromatin granules to the two daughter nuclei. It is indeed usually
assumed that the mitotic mechanism is not merely for the distribution,
but for the _equal_ distribution, of the sister granules to the two
daughter nuclei. The conspicuous part the chromatin is seen to play in
the whole mechanism of heredity--in maturation, fertilization and
development--indicating as it does that the chromatin is the chief, if
not the only, bearer of the specific qualities of the organism,
sufficiently clearly emphasizes the importance of the equal distribution
of this substance between the daughter cells at successive
cell-divisions. There are, however, serious objections to the
interpretation of mitosis as an adaptation to ensure this equal
distribution of the chromatin. Not only does the occurrence of amitosis
show that the mitotic mechanism is not essential for either nuclear or
cytoplasmic division, but direct division may occur[31] in the
life-history of the germ cells, the very point at which it should not
occur had mitosis the significance usually attached to it. On the other
hand, the most elaborate mitosis occurs in cell-tissues (e.g. skin of
salamander larva) which can take no possible share in the reproduction
of the species. Moreover, we have no reason for supposing that the
division of the chromatin in amitosis is not as meristic, and its
subsequent distribution as equal, as is so visibly the case in
mitosis.[32] It is necessary, therefore, to seek for some other
explanation of the elaborate mechanism of mitosis than that which
assumes it necessary for the equal distribution of the divided chromatin
granules. The present writer believes the true explanation to be found
in that great economic law of nature, "division of labour." The same
economy which, working under the control of natural selection, has
produced the complexly differentiated tissues of the higher metazoa,
which has led to the sexual differentiation between the conjugating
gametes and thus to the sexual differentiation of the parents, has
resulted in the production of mitosis. Only here the economy finds
expression in division of labour, not in space, but in time. The work of
the self-propagating chromatin granules is so ordered that periods of
undisturbed metabolic activity alternate with periods of reproductive
activity. The brief space of time occupied by the latter process has
necessitated a more elaborate specialization of the forces--whatever
their nature--controlling cell-division; a specialization which has
resulted, just as a similar specialization in so many other cases has
resulted, in a visible differentiation of the cell-protoplasm. This
explanation is in harmony with the occurrence of typical mitosis in
active tissue cells on the one hand, and of amitosis in the relatively
quiescent primary germ cells on the other.
_Individuality of the Chromosomes._--The most striking feature in the
behaviour of the chromatin in mitosis is its resolution, at each
division, into a--for any particular species--constant number of
chromosomes. This constant recurrence of the specific number of
chromosomes at every cell-division is capable of explanation in two
radically different ways. One explanation assumes for the organism a
specific peculiarity determining the segmentation of the spireme thread
into a definite number of segments (Delage, 1899 and 1901).[33] The
other regards chromosomes as independent units of the cell, retaining
their identity between successive cell-divisions. The latter
"Individualitats Hypothese" was originally put forward by Theodor Boveri
in 1887 as a result of C. Rabl's observation (1885) that in epidermal
cells of the salamander larva the chromosomes reappear in the mitosis of
the daughter cells with the same arrangement as they possessed in the
prophase of the mother cell--the angles of the U-shaped chromosomes
being all directed towards one pole (Rabl's "Poleseite") of the nucleus.
In the formation of the "resting" nucleus, the chromatin, becoming
metabolically active, flows out on to the linin reticulum, all trace of
the chromosomes being for the time lost. In _Ascaris_, Boveri (1888)
obtained similar but still more striking results. The thickened ends of
the four elongated chromosomes cause projections on the nuclear surface
throughout the resting period, and the ends of the reappearing
chromosomes always coincided with these protuberances; cf. also Sutton
(1902) on locust spermatagonia. Moreover, the arrangement of the
chromosomes must follow one of three well-marked groupings, and this is
determined for each individual in the cleavage spindle of the egg and
maintained throughout later development (fig. 8).
[Illustration: From Boveri's _Ergebnisse u. d. Konstitution der
chromatischen Substanz des Zellkerns_, by permission of Gustav Fischer.
FIG. 8.--Preparation for Mitosis. a, Nucleus of "1/2 blastomere" of
_Ascaris megalocephala bivalens_ in resting condition; b and c, nuclei
from sister 1/2 blastomeres in preparation for mitosis.]
In the same worm (var. _univalens_) Boveri (1888 and 1899) found that
occasional abnormalities in maturation resulted in the suppression of
the first polar body and the inclusion of its chromosomes in the second
maturation spindle; the egg-nucleus at the time of fertilization thus
having two chromosomes instead of one, while the spermatozoon nucleus
has only one. Three chromosomes instead of two reappear in subsequent
divisions. Boveri's "Individualitats Hypothese" received striking
support from the work of Herla (1893), L. R. Zoja (1895) and O. zur
Strassen (1898). Herla and Zoja showed that if the egg of _Ascaris
megalocephala_ (var. _bivalens_), which possesses two chromosomes, be
fertilized with the spermatozoon of var. _univalens_, in which the germ
cell has only one chromosome and that smaller than either of the two in
the other variety, three chromosomes reappear, two large and one small,
in the cleavage divisions of the resulting hybrid embryo. Zur Strassen's
observations on the giant embryos of _Ascaris_ also support Boveri's
theory. These embryos arise by the fusion of eggs, either before or
after fertilization. The number of chromosomes in the subsequent
cleavage-figures is proportional to the number of nuclei that have fused
together. Similar results are given by Boveri's (1893-1895) and T. H.
Morgan's (1895) experiments on the fertilization of enucleated
sea-urchin egg-fragments; all the nuclei of the resulting embryo having
only half the number of chromosomes characteristic of the species (e.g.
in _Echinus_ 9 instead of 18). All the above facts point to the
conclusion that, as Boveri expressed it in his _Grundgesetz der
Zahlenkonstanz_ (1888), "the number of chromosomes arising from a
resting nucleus is solely dependent on the number which originally
entered into its composition."[34]
_Boveri's Law of Proportional Nuclear Growth._--The chromatin in the
nucleus is exactly halved at every cell-division. As the bulk of the
chromatin remains constant from one cell-generation to another, it must
double its bulk between successive divisions. That this proportional
growth of the chromatin is dependent solely on the chromatin mass, and
not on that of the cell, is very clearly indicated by cases where the
normal chromatin mass has been artificially increased or reduced,[35]
the chromatin in either case doubling its bulk between successive
cell-divisions, and neither the mass of the chromatin nor the number of
the chromosomes undergoing any readjustment. By double or partial
fertilization, different regions in the same embryo may show nuclei of
different sizes (Boveri). We must therefore distinguish in the cell
between "young" and "adult" chromatin. In other words the chromatin must
be regarded as being composed of individual units, each with a definite
constant structure and maximum growth (Boveri, 1904). This conclusion is
strongly suggested, not only by the evidence in favour of the
individuality of the chromosomes considered above, but also by the
independent reproductive activity of the chromatin granules in the
prophase of mitosis.
[Illustration: From Boveri's _Ergebnisse u. d. Konstitution der
chromatischen Substanz des Zellkerns_, by permission of Gustav Fischer.
FIG. 9.--Preparation for Mitosis. a, Spermatogonium of _Brachystola
magna_ with resting nucleus; b, Same with prophase for mitosis. (After
Sutton.)]
_Differentiation among the Chromosomes._--If we grant the assumption of
a persistent individuality for the chromosomes, then it becomes possible
to consider whether in one and the same nucleus these structures may not
take varying parts in controlling the cell's activity in development and
in inheritance. Such a differentiation among the chromosomes would be
due to independent ancestry rather than to the economy resulting from a
division of labour; nevertheless a division of labour of a sort would be
the result of this gradual divergence of the chromosomes from one
another, and we might therefore expect that, in some cases at least, a
_morphological_ would accompany the _physiological_ differentiation.
Examples of such a morphological differentiation do indeed occur in the
"accessory" chromosomes first described by H. Henking (1891) for the
spermatogonia of _Pyrrhocoris_, and since described for numerous other
insects, Arachnids and Myriapods. W. Sutton's work on the
spermatogenesis of _Brachystola magna_ is of especial interest in this
connexion. Not only does the "accessory chromosome" in this insect form
a resting nucleus independent, and obviously physiologically
differentiated from that formed from the remaining chromosomes (fig. 9,
a), but the latter are themselves differentiated by size, there being
one pair of chromosomes of each size (fig. 9, b), a point of
considerable interest when we remember that half the chromosomes in each
cell are necessarily derived from each parent.[36]
Although this morphological differentiation among the chromosomes is
undoubtedly to be regarded as indicating a corresponding physiological
differentiation, it by no means follows that the latter need always, or
even generally, be accompanied by the former. Since, however, the
specific characters of the organism must be due to the combined activity
of _all_ the chromosomes, any physiological differentiation among the
latter should result in abnormal development if the full complement of
chromosomes be not present.[37] Boveri,[38] utilizing Herbst's
method[39] for separating echinoderm blastomeres, has interpreted in
this manner the abnormal development which H. Driesch[40] found almost
invariably to follow the double fertilization of the sea-urchin egg. In
such eggs the first cleavage spindle is four-poled. The chromosomes are
half again as numerous as in normally fertilized eggs (54 instead of
36), but each is only divided once, so that in the distribution of the
resulting 108 chromosomes the four daughter nuclei receive each only 27
instead of 36 (assuming the distribution to be fairly equal, which is by
no means usually the case in four-poled mitosis). Driesch had already
(1900) shown that any one of the first four blastomeres of a normally
fertilized egg will, if isolated, develop normally. Boveri found that in
the case of the doubly fertilized egg the isolated "1/4" blastomeres
develop very variously, a variability only to be accounted for by their
varying chromosome equipment. Occasionally a three-poled instead of a
four-poled figure resulted from double fertilization. In such cases
Driesch found, as we should expect from Boveri's interpretation, that
the percentage of approximately normal larvae was considerably greater;
for not only would the chances of an equal distribution of the
chromosomes be much greater, but the number received by each of the
three daughter cells would approximate to, or even equal, the normal.
_Reduction._--In all the Metazoa the prevailing, and in the higher forms
the only, method of reproduction is by the union (conjugation) of two
"sexually" differentiated germ-cells or "gametes"; a small motile
"microgamete" or spermatozoon and a large yolk-laden "macrogamete" or
ovum (see Reproduction). This differentiation between the germ-cells is
another example of the advantages of division of labour; for while the
onus of bringing about the union of the germ-cells is thrown entirely on
the spermatozoon, the egg devotes itself to the accumulation of
food-material (yolk) for the subsequent use of the developing embryo.
Far more yolk is thus secreted than would be possible by the combined
efforts of both the germ-cells had each of these at the same time to
preserve its motility. The fundamental physiological difference which
this division of labour has produced in the germ-cells is reflected on
to the general metabolism of the parents and underlies the sexual
differentiation of the latter.[41] Beyond this, however, sexual
differentiation does not go. The two germ nuclei which enter into the
formation of the first mitotic figure of the developing egg are not only
physiologically equivalent, but, at the time of their union in the egg,
are usually morphologically identical.[42] The essence of fertilization
is, therefore, the union of two germ nuclei only differing from one
another in that they are derived from separate individuals.[43] Since
the number of chromosomes appearing in mitosis is solely dependent on
the number which originally entered into the composition of the nucleus
(Boveri's Law of Chromosome-Constancy), it follows that, in the mitotic
figures of the developing embryo, the chromosomes will be half maternal,
half paternal in origin;[44] the germ nuclei thus necessarily possessing
only half the number of chromosomes characteristic of the ordinary
tissue cells of species, i.e. the somatic number.[45] The manner in
which this "reduction" in the number of chromosomes in the germ-cells is
brought about, and the significance to be attached to the process,
constitute the most hotly debated questions in cytology. In all the
metazoa the phenomenon of reduction is associated with the two last and,
usually, rapidly succeeding "maturation" divisions by which the
definitive germ-cells--ova or spermatozoa--are produced.[46]
Assuming the persistent individuality of the chromosomes, then there are
only three conceivable methods by which this numerical reduction can be
brought about (Boveri, 1904, p. 60). (1) One-half the chromosomes
degenerate. (2) The chromosomes are distributed entire, half to one
daughter cell, half to the other (reducing division of Weismann, 1887).
(3) The chromosomes fuse in pairs (_Conjugation of the Chromosomes_,
Boveri, 1892). The first possibility--that of an actual degeneration of
a part of the chromatin originally suggested by van Beneden and adopted
by August Weismann, Boveri and others, has been long abandoned, and a
steadily increasing bulk of evidence is tending to prove the general, if
not universal, occurrence of the second method--the distribution between
the daughter cells of undivided chromosomes. The occurrence of such a
"reducing division" was postulated on theoretical grounds by Weismann
(1887)[47] and by Boveri (1888); by the former as a result of his
adoption of de Vries's hypothesis of self-propagating and qualitatively
varying units for the chromatin; by the latter in relation to his theory
of chromosome individuality. The actual occurrence of this reducing
division was first demonstrated by Henking (1891) for _Pyrrhocoris_, and
afterwards by Hacker, vom Rath and many others, but especially by
Ruckert (1894) for _Cyclops_ (fig. 10). In this latter type the
chromatin of the oocyte, as this prepares for the first maturation
division, resolves itself into 12 (instead of 24) longitudinally split
chromosomes (fig. 10, a). As these continue to thicken and contract a
transverse fission appears (fig. 10, c). This is to be regarded as a
belated segmentation of the spireme thread, and shows that the reduction
so far is only a "pseudo-reduction" (Ruckert), the chromosomes being
really all present but temporally united in pairs, i.e. "bivalent"
(Hacker). A striking confirmation of this interpretation is provided by
Korschelt's description of reduction in the annelid _Ophryotrocha_. In
this type the full somatic number of split chromosomes (here only four)
appears, and these secondarily associate end to end in pairs, thus
forming split "diads" (i.e. tetrads), in every way similar to those
described by Ruckert for _Cyclops_. In the latter type, at the first
maturation division, the sister diads are separated from one another, an
"equating" division thus taking place. At the second division the diads
are resolved into their constituent parts, and the "univalent"
chromosomes are distributed to the daughter cells (reducing division).
A similar process has since been described for numerous other types
(e.g. various arthropods, Hacker, 1895-1898; vom Rath, 1895; and by
Sutton for _Brachystola_, 1902-1903). In _Ophryotrocha_, as in
_Pyrrhocoris_ (Henking), _Anasa_ (Paulmeir), _Peripatus_ (Montgomery),
&c., reduction occurs at the first maturation division ("pre-reduction"
of Korschelt and Heider, 1900), instead of at the second division
(post-reduction) as in most Copepods and Orthoptera. In many cases the
tetrads (i.e. split chromosomes associated in pairs) have the form of
rings, the genesis of which was first clearly determined by vom Rath
(1892) in the mole cricket _Gryllotalpa_ (fig. 11). In this form the
sister diads remain united by their ends but widely separate in the
middle (fig. 11, b). As in _Cyclops_, the belated transverse
segmentation appears as the condensation of the chromatin proceeds (fig.
11, d), but the symmetrical tetrads which this process here produces
make it impossible to determine at which of the two divisions reduction
is effected. An essentially similar ring formation occurs in _Enchaeta_
and _Calanus_ (vom Rath), and in the Copepods _Heterocope_ and Diaptomus
(Ruckert), and in other types.[48]
[Illustration: From Korschelt and Heider's _Lehrbuch d. vergl.
Entwicklungsgeschichte d. wirbellosen Tiere_, by permission of Gustav
Fischer.
FIG. 10.--Maturation Divisions. a-d, Formation of the tetrads in
_Cyclops_. (After Ruckert.) e, 1st maturation division; separation of
the bivalent sister chromosomes. f, 2nd maturation division;
distribution of the univalent chromosomes.]
[Illustration: From Prof. E. B. Wilson's _The Cell in Development and
Inheritance_, by permission of the author and of the Macmillan Co., N.
Y.
FIG. 11.--Maturation Divisions. Origin of the tetrads by ring formation
in the spermatogenesis of the mole-cricket (_Gryllotalpa_) (vom Rath).
a, Primary spermatocyte with six split, bivalent chromosomes. b and c,
Split has opened out. d, Concentration of the chromatin has made visible
the belated transverse division. e and f, Grouping of the completed
tetrads in the equatorial plate of the first maturation division.]
[Illustration: From O. Hertwig, _Allgemeine Biologie_, by permission of
Gustav Fischer.