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 theovary than in 
the spermary” (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 oégonia (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). 
