GEORGE T. HARGITT 



demonstrate that he often found the germ cells to be first recog- 

 nizable in the entoderm. The suggestion of an ectodermal 

 origin was proposed on theoretical grounds. But while the ecto- 

 dermal origin of the germ cells is proved not to be characteristic 

 of Hydrozoa, this does not necessarily discount the germ-plasm 

 theory. If it could be shown that the germ cells arise very early 

 in ontogeny and remain distinct and unchanged to the time of 

 sexual maturity and the formation of the gonads, it would be a 

 matter of no importance where these cells were located in the 

 interim, provided they remained passive and took no part in the 

 functioning of the body. 



In certain phyla considerable success has attended the inves- 

 tigation of germ-cell origin; an early differentiation has been 

 noted and these cells have been followed to their position in the 

 gonads. Some of these cases will be discussed later. In Hydro- 

 zoa, on the contrary, there has been an almost universal failure to 

 observe the differentiation at any time before sexual maturity 

 was reached. Weismann's studies were made on mature hydroids 

 and medusae, and only as a theoretical suggestion was an early 

 differentiation urged. Harm ('02), in young hydranths of Clava 

 squamata, just developing from planulae, found certain cells 

 which he believed to be primordial germ cells. These cells, 

 figured and described by Harm, are ectodermal cells similar in 

 form, size, and position to the interstitial cells; somewhat later 

 they form elongated, spindle-shaped cells lying directly against 

 the supporting membrane, and possessing a slightly more deeply 

 staining cytoplasm. They were not traced beyond this stage. 

 Wulfert ('02) traced the development of Gonothyraea loveni to 

 the formation of the polyp. While the planula is still within the 

 gonophore, interstitial cells are produced in the ectoderm and 

 entoderm, and these were followed through their differentiation 

 into ganglion cells and nematocysts. After the planula has 

 begun to transform into the polyp, Wulfert finds, for the first time, 

 what he believes to be germ cells. These occur in both ectoderm 

 and entoderm and, according to his figures, are like those cells of 

 an earlier period which became ganglion cells. Furthermore, the 

 cells called germ cells differ from other interstitial cells of all 

 stages only in their staining reaction. 



