GERM CELLS OF COELENTERATES 9 



tigators who have studied the development of Hydrozoa and 

 Scj^hozoa record the formation of germ layers and interstitial 

 cells and the differentiation of the latter into ganglion cells and 

 nematocysts, but have not observed the presence of germ cells 

 in these earlier stages. 



We may conclude that "Wulfert and Harm made no mistake 

 in their observations, but the interpretation of certain cells as 

 germ cells is not justified by their own evidence, nor is it confirmed 

 by this new study. The cells described as germ cells are inter- 

 stitial cells which were in the process of differentiation into 

 specialized cells of the body. In all the forms carefully studied 

 it is clear that germ cells do not occur in larvae or young polyps, 

 and in the absence of any e\ddence of their presence in similar 

 stages of other forms, there is ample reason for concluding that 

 an early differentiation of germ cells does not occur. 



The germ cells of Hydra have been investigated by a consid- 

 erable number of investigators, and practically all of these agree 

 upon the origin from interstitial cells of the ectoderm at the 

 breeding season. Brauer ('91) has observed the formation of the 

 interstitial cells before the ectoderm and entoderm are fully sepa- 

 rated, and has followed the differentiation of these into ganglion 

 cells and nematocysts, confirming the earlier results of Schneider 

 ('90). Dooming ('05, '09) is the only one who has suggested a 

 different conclusion for the germ cells of Hydra. He observes 

 the same origin from interstitial cells, but, in the developing 

 ovary, finds some interstitials to be larger than others; these he 

 beheves to be primordial germ cells which have been segregated 

 in early ontogeny to form a 'self propagating' germinal tissue. 

 He has not observed these cells in the embryo, indeed he seems 

 to have studied only the polyps which are producing reproductive 

 organs, and therefore his conclusions are largely hj^jothetical. 

 The presence of larger interstitial cells in the developing ovary and 

 their identification as germ cells is confirmed by Tannreuther ('08), 

 who also finds similar cells forming spermatogonia. But in every 

 case the formation of ovaries and spermaries is initiated by a 

 rapid growth of interstitial cells and later a multiplication of 

 these cells. Tannreuther thus accounts for the presence of larger 



