326 PEOFESSOR MARSHALL. 



zooid when this is formed, and later on shift again into the gonophore 

 which arises as an offset from this lateral branch. 



The explanation of this curious migration is probably to be found, 

 as Weismann suggests, in the advantage derived from commencing the 

 development of the sexual products as early as possible. The for- 

 mation of the ovum, especially, is a long and complicated process, 

 ■which in most animals is commenced at a very early date ; in the 

 highest mammals, for instance, the ovary contains either at or very 

 shortly after the time of birth all the ova that will ever be developed 

 in it. The development of spermatozoa is a more rapid and less 

 elaborate process than that of ova, and we find accordingly that the date 

 of their appearance is not thrown back so far as that of the ova. For 

 instance, in Eudendrium the ova arise in the primary zooid before the 

 appearance of the lateral branches ; the male cells, however, are not 

 formed till later, and appear first in the lateral branches, from which, 

 like the ova, they migrate into the gonophores. 



The suggestion I would make with regard to Hydra is that it repre- 

 sents one step further in the process of migration beyond the stage 

 reached by Cordylophora or Eudendrium; i.e. that in Hydra the 

 genital products not only make their first appearance in the wall of 

 the primary zooid, but remain and undergo their whole development 

 in the same position, no lateral bud or gonophore being formed. 



Weismann 1 himself takes the directly opposite view that Hydra 

 represents a primitive, and not, as I believe it to be, an extremely modi- 

 fied condition. He considers that in Hydra there has been no shifting 

 of the place of origin of the sexual cells, but that Hydra represents 

 in this respect the primitive and original condition. 



In support of the contention that Hydra is, as regards its genera- 

 tive organs, a modified and not a primitive form I would submit the 

 following arguments : — 



1. Hydra is hermaphrodite, being in this respect almost unique 

 among Hydrozoa. A hermaphrodite condition is altogether excep- 

 tional among Hydrozoa, and there is not the slightest evidence for 

 regarding it as primitive in them; while there is very strong reason for 

 viewing it as secondary and acquired, wherever it occurs in other 

 groups of animals. 



2. Hydra is fresh water, differing in this respect also from almost all 

 other Hydrozoa. It is very generally accepted that fresh water forms 



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



