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,[1] 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.
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From Boveri’s Ergebnisse ü. 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.[2]
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.[3] Boveri,[4] utilizing Herbst’s method[5] for separating echinoderm blastomeres, has interpreted in this manner the abnormal development which H. Driesch[6] 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.[7] 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.[8] 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.[9] Since the number of chromosomes appearing in mitosis is solely dependent on the number which
- ↑ Boveri (1902), “Fertilization of enucleated Echinus-egg fragments,” and M. Boveri (1903); by shaking the egg shortly after fertilization the sperm centrosome is prevented from dividing, and a monaster instead of a diaster results, the divided chromosomes remaining in the one nucleus.
- ↑ Cf. especially in this connexion Häcker’s paper Über die Schicksale der elterlichen und grosselterlichen Kernanteile (1902).
- ↑ Each nucleus contains a duplicate set of chromosomes, the one of maternal, the other of paternal origin, and either of these sets alone suffices for development. This is clearly shown by the experiments of Loeb (1899) and Wilson (1901) on the artificial parthenogenesis of the sea-urchin egg; and those of O. Hertwig (1889 and 1895), Delage (1899) and Winkler (1901), on the fertilization of enucleated Echinoderm eggs (Merogony, Delage). The fact that in some forms, e.g. Ascaris megalocephala var. univalens, only one chromosome is derived from each parent, originally led Boveri to conclude that all chromosomes must necessarily be physiologically equivalent.
- ↑ Über mehrpolige Mitosen als Mittel zur Analyse des Zellkerns (1902).
- ↑ Über das Auseinandergehen von Furchungs- und Gewebezellen in kalkfreien Medium (1900).
- ↑ “Entwicklungsmechanische Studien V.” (Zeit. für wiss. Zool., Bd. lv., 1892).
- ↑ See Geddes and Thomson, Sex, esp. pp. 127, 137 and 139.
- ↑ The equivalence of the germ nuclei in development is shown by the experiments on the fertilization of enucleated eggs and artificial parthenogenesis already referred to.
- ↑ O. Hertwig, 1873; but esp. van Beneden, 1883.
