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 “¼” 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 Häcker, vom Rath and many others, but especially by Rückert (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” (Rückert), the chromosomes being really all present but temporally united in pairs, i.e. “bivalent” (Häcker). 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 Rückert 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, Häcker, 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 (Rückert), and in other types.[48]
| 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 Rückert.) e, 1st maturation division; separation
of the bivalent sister chromosomes. f, 2nd maturation division;
distribution of the univalent chromosomes. |
| 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. |
| From O. Hertwig, Allgemeine Biologie, by permission of Gustav Fischer. |
| Fig. 12.—Heterotypical Mitosis. (Schematic, after Flemming.) |
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” (Häcker, 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 Rückert, 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 (συνάπτειν, 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]