742 CCELENTERATA. 



The following apparent modes of origin of these products 

 have been observed. 



(1) The generative products of both sexes originate in the 

 ectoderm (epiblast) : Hydra, Cordylophora, Tubularia, all (?) free 

 Gonophores of Hydromedusae, the Siphonophora, and probably 

 the Ctenophora. 



(2) The generative products of both sexes originate in the 

 entoderm (hypoblast) : Plumularia and Sertularella, amongst 

 the Hydroids, and the. whole of the Acraspeda and Actinozoa. 



(3) The male cells are formed in the ectoderm, and the 

 female in the entoderm : Gonothyraea, Campanularia, Hydrac- 

 tinia, Clava. 



In view of the somewhat surprising results to which the 

 researches on the origin of the genital products amongst the 

 Ccelenterata have led, it would seem to be necessary either to 

 hold that there is no definite homology between the germinal 

 layers in the different forms of Ccelenterata, or to offer some 

 satisfactory explanation of the behaviour of the genital pro- 

 ducts, which would not involve the acceptance of the first 

 alternative. 



Though it can hardly be said that such an explanation has 

 yet been offered, some observations of Kleinenberg (No. 557) 

 undoubtedly point to such an explanation being possible. 



Kleinenberg has shewn that in Eudendrium the ova migrate 

 freely from the ectoderm into the endoderm, and vice versa ; but 

 he has given strong grounds for thinking that they originate in 

 the ectoderm. He has further shewn that the migration in this 

 type is by no means an isolated phenomenon. 



Since it is usually only possible to recognise generative 

 elements after they have advanced considerably in development, 

 the mere position of a generative cell, when first observed, can 

 afford, after what Kleinenberg has shewn, no absolute proof 

 of its origin. Thus it is quite possible that there is really 

 only one type of origin for the generative cells in the Ccelen- 

 terata. 



Kleinenberg has given reasons for thinking that the migration of the ova 

 into the entoderm may have a nutritive object. If this be so, and there are 

 numerous facts which shew that the position of generative cells is often 

 largely influenced by their nutritive requirements, it seems not impossible 



