PORIFERA, CCELENTERATA, VERTEBRATA 83 



in 1872, upon which most of the accounts in our 

 zoological textbooks are still based. Kleinenberg's 

 researches were followed by those of Korotneff 

 (1883), Nussbaum (1887), Schneider (1890), and 

 Brauer (1891). Investigations of the germ cells of 

 Hydra then almost ceased until 1904, when another 

 period of activity in this field began and papers 

 quickly followed one another (Guenther, 1904 ; 

 Downing, 1905; Hadzi, 1906; Hertwig, R., 1906; 

 Tannreuther, 1908, 1909; Downing, 1909; and 

 Wager, 1909). The following account is based 

 chiefly upon the researches of Downing (1905, 

 1908, 1909), Tannreuther (1908, 1909), and Wager 

 (1909). 



The origin of the male germ cells has been carefully 

 investigated by Downing (1905) and Tannreuther 

 (1909). Previous to Downing's researches all in- 

 vestigators, beginning with Kleinenberg (1872), 

 considered the sex cells as interstitial in origin. 

 Downing, however, believes that -germ cells and in- 

 terstitial cells may be distinct. The sex cells, 

 according to this investigator, are distinguished 

 "by their very large nuclei, extremely granular, 

 and often by the presence of a Nebenkern " (Fig. 

 27, C). "The characters of the sex cells . . . 

 seem constant, and my conclusion would be that at 

 some stage of the embryonic development certain 

 cells are stamped with these characters and that they 

 and their progeny form the sex cells distinct through- 

 out the life of the individual . . . the germ-plasm is 

 then continuous in Hydra " (p. 413). This tentative 



