ORIGIN AND GROWTH OF THE GERM-CELLS III 



only by its size, but also by its large nuclei rich in chromatin, and by 

 its peculiar mode of mitosis, as described beyond. 



The most beautiful and remarkable known case of early differenti- 

 ation of the germ-cells is that of Ascaris, where Boveri was able to 

 trace them back continuously through all the cleavage-stages to the 

 two-cell stage! Moreover, from the outset the progenitor of the germ- 

 cells differs from the somatic cells not only in the greater size and richness 

 of chromatin of its nuclei, but also in its mode of mitosis ; for in all 

 those blastomeres destined to produce somatic cells a portion of the 

 chromatin is cast out into the cytoplasm, where it degenerates, and 

 only in the germ-cells is the sum total of the chromatin retained. In 

 Ascaris megalocephala nnivalens the process is as follows (Fig. 55): 

 Each of the first two cells receives two elongated chromosomes. As 

 the ovum prepares for the second cleavage, the two chromosomes 

 reappear in each, but differ in their behaviour (Fig. 55, A, B). In one 

 of them, which is destined to produce only somatic cells, the thickened 

 ends of each chromosome are cast off into the cytoplasm and degen- 

 erate. Only the thinner central part is retained and distributed to 

 the daughter-cells, breaking up meanwhile into a large number of 

 segments which split lengthwise in the usual manner. In the 

 other cell, which may be called the stem-cell (Fig. 55, s), all the 

 chromatin is preserv^ed and the chromosomes do not segment into 

 smaller pieces. The results are plainly apparent in the 4-cell stage, 

 the two somatic nuclei, which contain the reduced amount of chro- 

 matin, being small and pale, while those of the two stem-cells are far 

 larger and richer in chromatin (Fig. 55, C). At the ensuing division 

 (Fig. 55, D) the numerous minute segments reappear in the two 

 somatic cells, divide, and are distributed like ordinary chromosomes ; 

 and the same is true of all their descendants thenceforward. The 

 other two cells (containing the large nuclei) exactly repeat the 

 hi.story of the two-cell stage, the two long chromosomes reappearing 

 in each of them, becoming segmented and casting off their ends 

 in one, but remaining intact in the other, which gives rise to two 

 cells with large nuclei as before. This process is repeated five 

 times (Boveri), or six (Zur Strassen), after which the chromatin- 

 elimination ceases, and the two stem-cells or primordial germ-cells 

 thenceforward give rise only to other germ-cells and the entire 

 chromatin is preserved. Through this remarkable process it comes 

 to pass that in this animal only the germ-cells receive .the sum 

 total of tJie egg-chromatin handed doivn from the parent. All of the 

 somatic cells contain only a portion of the original germ-substance. 

 "The original nuclear constitution of the fertilized ^^2, is transmitted, 

 as if by a law of primogeniture, only to one daughter-cell, and by this 

 again to one, and so on ; while in the other daughter-cells, the 



