324 SENESCENCE AND REJUVENESCENCE 



and sometimes spermatozoa and eggs approaching in morphological 

 differentiation those of the multicellular forms appear. In the 

 multicellular animals the process of gamete formation differs in 

 certain respects from that in the plant. There is in the animal no 

 developmental history with cell division, growth, and differentiation 

 between maturation and fertilization, corresponding to the gameto- 

 phyte generation in plants. The gametic cells are segregated from 

 other cells long before the maturation divisions occur. Since the 

 germ-plasm theory has found its adherents chiefly among zoologists, 

 it is natural that the attention of zoological investigators should 

 have been attracted to the question of the early segregation of the 

 germ cells from the somatic cells. If the germ plasm is really a 

 distinct and separate entity independent of the soma and is con- 

 tinuous from one generation to another, we should expect the germ 

 cells to be segregated from the somatic cells at the beginning of 

 embryonic development. Thus far, however, no case has been 

 discovered in which such a segregation occurs, although in various 

 animal groups a more or less complete segregation apparently does 

 occur at an early stage of development. In other groups, among 

 the animals, no indication of such segregation has ever been ob- 

 served, although theoretical considerations have led many zoolo- 

 gists to believe that even in such cases a segregation occurs, but 

 without visible differences between germ cells and other cells. 



To discuss this subject at length is beyond the present purpose, 

 but some of the more important cases of early segregation must be 

 briefly considered. 1 Perhaps the most striking case of early segre- 

 gation of germ cells is that in the parasitic worm Ascaris megalo- 

 cephala, first described by Boveri and later confirmed by other 

 investigators, but recently denied by Zacharias. 2 As every zoolo- 

 gist knows, the process of segregation of the germ cells in this 

 species begins at the first cleavage of the egg and is accompanied 

 by the peculiar process of "diminution'' of the chromatin in the 

 somatic cells. Diminution, which occurs first in one cell of the 



1 For general surveys of the subject with bibliographies see Korscheldt and 

 Heider, '02, pp. 368-77; Waldeyer, '06; Felix and Biihler, '06; Hacker, '120, '126; 

 Hegner, '14^. 



2 Boveri, '87, '99, '04; zur Strassen, '96; Zacharias, '13; Zoja, '96. 



