10 GEORGE T. HARGITT 



interstitial cells, but shows they are not a germinal tissue. He 

 finds no such cells before or after the formation of the repro- 

 ductive organs and can trace their growth from ordinary inter- 

 stitial cells. Later, Tannreuther ('09) followed the behavior of 

 the interstitial cells and could find all gradations between small 

 and large interstitials, as well as trace the transformation of an 

 ordinary interstitial through spermatogonia into spermatozoa. 



Wager ('09) finds no evidence of a germinal tissue in Hydra, 

 nor of any difference between interstitial cells. Furthermore, in 

 the very groups of interstitial cells which grow^ to form oogonia, 

 ''one usually finds nematocysts developing in large numbers. In 

 the course of development of the ovarian area these nematocysts 

 either migrate out or are resorbed. Frequently they are found 

 within the egg itself." This is a very striking demonstration 

 of the equipotency of the interstitial cells and effectually refutes 

 the belief of a distinct germinal tissue composed of certain inter- 

 stitial cells. The characters used by Downing to differentiate 

 germ cells from other interstitials are found to be applicable to 

 most interstitial cells; there is great variety in size and appear- 

 ance, and Wager finds all gradations between these variations 

 in interstitial cells. He strongly confirms the work of the earlier 

 authors and agrees perfectly with Tannreuther in the absence of 

 a distinct germinal tissue in Hydra. The work of these two 

 authors did not include a complete study of the histogenesis, 

 but the investigations of Schneider ('90) and Brauer ('91) com- 

 pletely fill this gap. Hegner ('14), in discussing the germ cells 

 of Hydra, says he ''is inclined to accept Downing's position in 

 the matter." But Downing's position is untenable, for his 

 conclusions are refuted by the work of other investigators. The 

 strongest evidence of the occurrence of a distinct germinal tissue 

 presented by Downing, viz., the presence of larger interstitial 

 cells of a distinct sort, is shown by both Wager and Tannreuther 

 to be merely an incident in the formation of reproductive organs. 

 From the facts presented in the above discussion there is but 

 one conclusion which may fairly be drawn, viz., in Hydra and 

 other Hydrozoa there is no clear evidence that germ cells are ever 

 differentiated in larvae, young polyps, or any early stage in 

 ontogeny. 



