PORIFERA, CCELENTERATA, VERTEBRATA 89 



One more Hydrozoon may be mentioned — 

 Gonothyrcea loveni — since Wulfert (1902) traced 

 the germ cells of this species back to the planula 

 stage where they arise from the interstitial cells of 

 the ectoderm and later undergo characteristic 

 migrations. 



Our knowledge of the origin of the germ cells 

 in other coelenterates is very fragmentary and even 

 less decisive than that of the Hydrozoa. For this 

 reason a consideration of the subject is omitted here. 



Discussion. As in the Porifera we are here 

 confronted with the question whether or not there is 

 continuity of the germ-plasm in the Ccelenterata. 

 There is sufficient evidence for the belief that the 

 cells which develop into germ cells are not derived 

 from the ectoderm or the entoderm but belong to a 

 special sort of propagative cells which are scattered 

 about among the other cells throughout the body 

 and which give rise to ova or spermatozoa under 

 certain environmental conditions differing in the 

 different species. This conclusion is based partly 

 upon the results of Downing (1905, 1908, 1909), 

 who still holds, as stated in his published papers, 

 that there is continuity of the germ-plasm in Hydra ; 

 and upon the fact that germ cells have been recog- 

 nized in the young hydranths of Clava (Harm, 1902) 

 and in the planula of Gonothyroea (Wulfert, 1902). 

 It seems certain that more careful studies of the 

 early stages of coelenterates with special regard 

 to the origin of the germ cells and with the use of 

 many and varied stains would result in the discovery 



