46 GEORGE T. HARGITT 



The germ cells of some animals have been observed to form 

 relatively early in ontogeny, but such is not the case in coelen- 

 terates. It has been claimed by some writers that in Hydra 

 and a few hydroids germ cells are differentiated in the larvae. 

 This claim has been refuted by later studies upon Hydra, and 

 a new investigation of larval stages of hydroids furnishes no 

 evidence of an early differentiation of germ cells in those forms. 

 Furthermore, there are a number of the Hydrozoa whose germ 

 cells have been observed to arise directly from differentiated 

 body cells. This happens either by the transformation of an 

 entire epithelial cell into a germ cell or by the division of a body 

 cell and the transformation of one of these division products 

 into a germ cell. In the latter case the sister cell persists as a 

 functional tissue cell. In at least ten species of eight different 

 genera the germ cells have been observed to form in this way. 

 This positive evidence, together with the refutation of all con- 

 trary claims, points to a single conclusion, viz., in the Hydrozoa 

 (probably also in all coelenterates) germ cells are not dif- 

 ferentiated in early ontogeny, but only much later as the time 

 of sexual maturity is at hand. 



The theory of the continuity of the germ plasm postulates 

 the formation of a somatic blastomere and a germinal blasto- 

 mere at the first cleavage of the egg ; in no animal is such a result 

 known. As applied to hydroids, the theory originally admitted 

 the origin of germ cells from histologically differentiated somatic 

 cells, but invoked the aid of invisible and unrecognizable germ 

 substance lying latent in such body cells. The production of a 

 germ cell by the transformation of half a tissue cell, and the 

 persistence of the other half as a tissue cell, is sufficient to 

 disprove the claim of the presence of an invisible germ plasm in 

 such tissue cells. This fact, together with the origin of germ 

 cells only as sexual maturity approaches, indicates a lack of 

 continuity of the germ plasm in the coelenterates. 



As explained by the germ-plasm theory, budding is always due 

 to the presence of latent germ cells. But budding in Hydra 

 and hydroids involves the activity of all the layers of the 

 budding zone; in Hydra it is possible that interstitial cells first 



