PORIFERA, COELENTERATA, VERTEBRATA 83 
in 1872, upon which most of the accounts in our 
zoodlogical 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 theindividual . . . the germ-plasm is 
then continuous in Hydra” (p. 413). This tentative 
