328 PROFESSOR MARSHALL. 



the plate, develops into an ovum, the remaining cells supplying it 

 with nutriment. This must be regarded as a very highly specialised 

 condition, for all the cells of the ovary are at first alike and must be 

 supposed to be of equal value. It very usually happens among ani- 

 mals that of the cells composing the ovary, certain ones alone develop 

 into ova, the others serving to feed them, but it is altogether excep- 

 tional that only a single ovum should attain maturity : other instances 

 are indeed known, such as Moina and perhaps some other Entomos- 

 traca, and also some Ascidians, such as Salpa, but such cases are rare. 



I am disposed to lay stress on this point, for if it be granted, as I 

 think it must, that the ovary of Hydra is in an exceptionally special- 

 ised condition, it becomes very difficult to believe, as Weismann would 

 have us do, that it is also from another point of view in a far more 

 primitive condition than that of other Hydroids. 



5. Kleinenberg and others have denied that any direct comparison 

 is possible between the reproductive organs of Hydra and the gono- 

 phores of an ordinary Hydroid on the ground that the former consist 

 of ectoderm alone, while the latter even in their most degenerate con- 

 dition involve endoderm as well as ectoderm ; and it must be admitted 

 at once that the objection is a valid one. 



However, it does not in the least affect the position I have attempted 

 to establish, which is that the reproductive organs of Hydra correspond 

 not to the gonophore of Cordylophora but to the zone of germination 

 round the necks of the zooicls in which the genital cells arise in Cordy- 

 lophora and in which they both arise and ripen in Hydra. 



I would in conclusion point out that the above argument only con- 

 cerns the sexual organs of Hydra. 



As regards its general morphology I fully agree with Weismann 1 

 that Hydra has strong claims to be regarded as having departed very 

 little from the condition of the ancestral form from which all Hydro- 

 medusae may have sprung. 



1 Weismann, op. cit., p. 254. 



