84 GERM-CELL CYCLE IN ANIMALS 



opinion is expressed with more certainty in a later 

 paper (Downing, 1909), since the "distinctive charac- 

 ter of the germ cell is more marked in the ovary than in 

 thespermary" (p. 311). Tannreuther (1909), on 

 the other hand, claims that the male germ cells are 

 interstitial in origin, and " the progenitors of the 

 spermatozoa have no special characters by which 

 they can be recognized as germ cells." 



The origin of the eggs of Hydra is better known 

 than that of the male germ cells. The ova have by 

 most investigators been considered modified intersti- 

 tial cells. Downing (1908, 1909) disagrees in several 

 respects with the results of Tannreuther and Wager. 

 His most important difference is regarding the ques- 

 tion of the origin of the ova directly from interstitial 

 cells or from definite propagative cells that are set 

 aside for reproductive purposes at some stage in the 

 animal's embryonic development. He believes " that 

 in the adult Hydra the oogonia (and spermatogonia) 

 are distinctly differentiated as a self-propagating 

 tissue" (p. 310). Wager (1909), on the contrary, 

 claims that it is impossible to prove that eggs do 

 not arise from ordinary interstitial cells; whereas 

 Tannreuther (1909) finds that the primitive ova can 

 be distinguished from interstitial cells "by their 

 large nucleus, nucleolus, and abundance of chromatin, 

 even before the growth of the ovary begins" (p. 205), 

 especially during the breeding season, and admits 

 that "If these sex cells could be distinguished during 

 the budding season as well, it would at least suggest 

 specificity of the germ cells " (p. 205). 



