148 THE GERM-CELLS 



the ovum prepares for the second cleavage, the two chromosomes 

 reappear in each, but differ in their behaviour (Fig. 73, 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. 73, s\ all the 

 chromatin is preserved and the chromosomes do not segment into 

 smaller pieces. The results are plainly apparent in the four-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. 73, C). At the ensuing division 

 (Fig. 73, 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 

 history 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 the egg-cJiromatin handed dozvn from the parent. All of the 

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

 " The original nuclear constitution of the fertilized o^gg 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 

 chromatin in part degenerates, in part is transformed, so that all of 

 the descendants of these side-branches receive small reduced nuclei." ^ 



It would be difficult to overestimate the importance of this dis- 

 covery ; for although it stands at present an almost isolated case, yet 

 it gives us, as I believe, the key to a true theory of differentiation 

 development,^ and may in the end prove the means of explaining 

 many phenomena that are now among the unsolved riddles of the cell. 



Hacker ('95) has shown that the nuclear changes in the stem- 

 cells and primordial eggs of Cyclops show some analogy to those of 

 Ascaris, though no casting out of chromatin occurs. The nuclei are 

 very large and rich in chromatin as compared with the somatic cells, 

 and the number of chromosomes, though not precisely determined, 



1 Boveri, '91, p. 437. ^ Cf. p. 426. 



