All the above cases, in which the reduction is effected by the
distribution of entire chromosomes at one or other of the maturation
divisions, may be grouped together as "pseudomitotic" (Hacker, and
Korschelt & Heider). In sharp contrast to the pseudomitotic method is
the "Eumitotic" method, in which the chromosomes are longitudinally
divided at both divisions. Such a method not only robs the process of
any "reducing" value in Weismann's sense, but is in serious conflict
with the chromosome-individuality hypothesis. Nevertheless it is in this
sense that Boveri (1881) and van Beneden (1883-1887) described the
maturation of the egg, and at a later period Brauer (1893) that of the
spermatozoon, in _Ascaris_. In each case the tetrads are formed by the
double longitudinal splitting of the chromosomes, the latter appearing
in the prophase in the reduced number. Not only was the eumitotic method
of _Ascaris_ the first method to be described, but the descriptions are
fully equal in point of clearness to that of Hertwig for the
pseudomitotic maturation of _Cyclops_.[49] A similar eumitotic
maturation has been described for other types also, e.g. _Sagitta_ and
the Heteropods, but nowhere more frequently than in the Vertebrates
among animals and the Phanerogams among plants. In these two latter
groups the chromosomes of the reducing division only rarely have a ring
form comparable to that seen in _Gryllotalpa_, &c. When such rings do
occur their genesis is very obscure, and at no time do they present the
appearance of "tetrads." It is the characteristic appearance these
looped chromosomes give to the first maturation division in many
Vertebrates, and especially in the Amphibia (fig. 12), that originally
led Flemming (1887) to term this type of mitosis "heterotypical"; the
second division, lacking this peculiar appearance, being distinguished
as "homotypical." Until quite recently these looped chromosomes of the
heterotypical mitosis of Vertebrates (and plants) were described as
arising by the opening out of longitudinally split chromosomes, exactly
as this occurs in the early prophase of the maturation divisions in such
types as _Gryllotalpa_, _Diaptomus_, &c. In the heterotype mitosis,
however, no transverse segmentation appears, and the halves of the
rings, as they separate in the first division, show an obvious
longitudinal split in preparation for the second division.[50] Both
divisions were thus interpreted as equating divisions.[51] The more
recent works of Farmer and Moore (1903-1905), Montgomery (1903,
Amphibia), and (for plants) Strasburger (1903-1904) have shown, however,
that even for the higher plants and animals, a reducing division in
Weismann's sense occurs in an essentially similar manner to that so
convincingly described by Ruckert, vom Rath and others, for
Invertebrate types. For the chromosomes of the heterotype mitosis arise
by the looping round, not opening out, of the bivalent chromosomes. The
first division is thus a reducing division, while the split appearing in
the anaphase of the heterotype and presumably reappearing in the
prophase of the homotype is the original split of the spireme thread.
The widespread, if not universal, formation of tetrads, i.e. the
temporary union in pairs of split chromosomes, in reduction, and the
relation this latter process always bears to _two_ rapidly succeeding
maturation divisions--those completing the gametogenic cycle in animals
and terminating the sporophytic generation in plants,--has received a
suggestive explanation at the hands of Boveri (1904). The growth of the
chromatin is an indispensable prelude to its reproduction (Boveri's Law
of Proportional Growth). The chromatin is therefore incapable of
undergoing reproductive fission in two successive mitotic divisions when
these are not separated by a resting (i.e. growth) period. In addition
to this, the "bipolar" condition of the adult chromosomes, which
determines its mode of attachment to mantle fibres from _both_ poles of
the spindle, is not possessed by the unripe chromatin. The undivided,
i.e. unripe, chromosomes are therefore incapable of utilizing the
mitotic mechanism for such a transverse fission as Weismann originally
postulated. The difficulty is, however, at once overcome if the unripe
chromosomes are associated in pairs in the equatorial plate, for the
bivalent chromosomes so produced are bipolar just as are the adult (i.e.
split) chromosomes in the ordinary and homotype mitosis.[52]
_Synopsis_ ([Greek: synaptein], to fuse together).--During the prophase
of the reducing or heterotype divisions the whole of the chromatin
becomes temporarily massed together at one pole of the nucleus (Moore,
1896, for Elasmobranchs). Montgomery (1901) has suggested that this is
to facilitate the temporary union in pairs, or "conjugation" of
homologous paternal and maternal chromosomes. In _Ascaris megalocephala_
var. _univalens_, where the somatic number is only two, the association
must necessarily be between homologous chromosomes. The assumption that
this "selective pairing" of equivalent chromosomes is universal is
supported by the behaviour of the "Heterochromosomes" (Montgomery) of
the Hemiptera. These chromosomes, distinguished by their size, are
paired before, and single after, the "pseudo-reduction" has taken place.
Even more convincing is Sutton's account of reduction in _Brachystola_
already referred to.[53] Boveri (1904) has suggested that this temporary
association of the chromosomes--presumably facilitated by the
synapsis--has a much deeper meaning than to ensure their correct
distribution between the daughter nuclei in the heterotype mitosis; the
associated chromosomes exchanging material in a manner analogous to
conjugation in _Paramoecium_.[54]
_Present Position of the Cell-theory._--Since the time of Schleiden and
Schwann a wealth of evidence has accumulated in support of the
"cell-theory"--the theory which regards the cell as the unit of organic
structure. "The organism consists morphologically, of cells, and
subsists, physiologically, by means of the 'reciprocal action' of the
cells,"--this was the cell standpoint of Schleiden and Schwann, and it
is no exaggeration to say that this same conception has dominated the
cell-theory almost to the present day.[55] The frequently striking
correlation between cell-division and cell-differentiation in
development has caused this process to be regarded as dependent on
cell-division, while a wholly exaggerated importance has been attached
to the distinction between "unicellular" and "multicellular"
organisms--between "intercellular" and "intracellular" organs. The
influence of the "cells" upon one another, the subordination of the
cell's growth, division and differentiation, to the requirements of the
whole organism--seen in normal growth, but nowhere more strikingly than
in development and regeneration,--is, however, very difficult of
explanation in terms of the cell-theory as this was, until quite
recently, generally understood. The very elaborate regional
differentiation of the protoplasm often seen in the Protozoa
sufficiently indicate that multicellular structure is no essential
condition for complex regional differentiation. That the regional
differentiation of the protoplasm in the Metazoa should usually
correspond with cell-limits is scarcely surprising. Nor is it to be
wondered at that, with so convenient a mechanism for segregation to hand
as cell-division, the progressive differentiation seen during
development should often appear to go hand in hand with this process. In
recent years, however, evidence has been steadily accumulating to show
that this association between cell-division and regional differentiation
of the protoplasm in development is a casual one--as casual, and as
natural, as the correspondence between cell limits and regional
differentiation in the formed tissues. The fact that the regional
differentiation may be foreshadowed in the egg before cleavage
begins,[56]--that as Driesch has shown, the mode of cleavage may be
artificially altered without affecting the ultimate organization of the
embryo,--and many other similar observations, tend to emphasize the
importance of the "organism" standpoint (C. O. Whitman, 1903, p. 642) in
contradistinction to the widely prevalent "cell" standpoint. The
occurrence of syncytial organs and organisms, and the increasing
frequency with which protoplasmic continuity is being demonstrated
between all kinds of cells, are facts tending in the same direction. In
the plant kingdom the growth of the _mass_ has been recognized as the
primary factor in development;[57] _die Pflanze bildet Zellen, nicht die
Zelle bildet Pflanzen_ (de Bary). For the animal kingdom this
"Inadequacy of the Cell-Theory of Development" has been maintained
amongst others by Whitman,[58] and by Adam Sedgwick.[59] The latter
author, mainly as the result of work on the development of _Peripatus_
and of Elasmobranch embryos, regards the developing embryo as a
continuous protoplasmic reticulum, for the nuclei of which the limiting
epithelial layers constitute as it were a breeding ground.
Differentiation is a regional specialization of this nucleated meshwork,
and is not to be regarded as the result of the proliferation and
subsequent specialization of cells predestined by cleavage for this end.
It is possible to suggest a mechanico-physical explanation of
multicellular structure which will deprive the cell of much of its
assumed significance as a unit of organization. The fact that surface
area becomes relatively less extensive as bulk increases would alone set
a limit to the size of "unicellular" organisms; for not only is there a
constant reaction between nucleus and cytoplasm through the nuclear
membrane, but the surface of the cell serves both for the intake of food
and the elimination of waste material. In addition to the limit thus
imposed upon the cytoplasmic area which can be effectually controlled by
the nucleus, and the necessity for a minimum surface area to the
protoplasmic mass, the advantages of the more or less complete
subdivision of the living substance into--as far as their metabolism is
concerned--semi-autonomous units, is indicated by the mechanical support
derived from the specialized cell walls and turgescent cells of the
plant, and the intercellular secretions of the animal tissues. It is
more than possible that these two conditions--i.e. surface area for
diffusion, and mechanical support--are alone responsible for the
_origin_ of multicellular structure, and that the sharply defined
character this now so generally possesses has been secondarily acquired
as a result of the facilities it undoubtedly offers for regional
specialization in the protoplasmic mass.
BIBLIOGRAPHY.--The special literature of cytology has grown to large
dimensions. The following are the more important text-books and papers
of general interest: E. B. Wilson, _The Cell in Development and
Inheritance_ (2nd ed., 1900); A. Gurwitsch, _Morphologie und Biologie
der Zelle_ (Jena, 1904); O. Hertwig, _Allgemeine Biologie_ (Jena,
1906); Korschelt and Heider, _Lehrbuch der vergl.
Entwicklungsgeschichte der wirbellosen Tiere_, Allgem. Teil, "The Germ
Cells and Experimental Embryology" (Jena, 1903); Whitman, "The
Inadequacy of the Cell Theory of Development," _Journ. Morph._ viii.,
1893; Adam Sedgwick, "On the Inadequacy of the Cellular Theory of
Development," _Quart. Journ. Micro. Science_, xxxvii.; G. C. Bourne,
"A Criticism of the Cell Theory" (an answer to Sedgwick's paper),
_Quart. Journ. Micro. Science_, xxxviii.; Th. Boveri, "Befruchtung,"
_Merkel-Bonnets Ergebnisse der Anat. u. Entwicklungsgesch._ Bd. i.
(1892), _Das Problem der Befruchtung_ (Jena, 1902), _Ergebnisse uber
die Konstitution der chromatischen Substanz des Zellkerns_ (Jena,
1904); J. Ruckert, "Die Chromatinreduktion bei der Reifung der
Sexualzellen," _Merkel-Bonnets Ergebnisse_, Bd. iii. (1894); V.
Hacker, "Die Reifungserscheinungen," _Ergebn. Anat. u.
Entwicklungsgesch._ Bd. viii. (1898); F. Meves, "Zellteilung,"
_Merkel-Bonnets Ergebnisse_, Bd. viii. (1898, 1899); W. Waldeyer, "Die
Geschlechtszellen," in O. Hertwig's _Handbuch der vergleich. u.
experiment. Entwicklungslehre d. Wirbeltiere_ (1901, 1903).
(G. C. C.)