35 



water genus Cordylopliora, in which the ova arise in what 

 Weismann calls the gei'minal zone of the primary zooid, 

 then migrate into the lateral branch of the 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 development of the ovum, espe- 

 ciall}^, 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 Lhe lateral 

 branches, from which, like the ova, they migrate into the 

 gonophores. 



The suggestion I would make with regard to Hydra is 

 that it represents one step further in the process of migra- 

 tion beyond the stage reached by Cordylophora or Euden- 

 drium ; 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 himself takes the direct opposite view that 

 Hydra represents a primitive, and not, as I believe it to be, 

 an extremely modified 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. 



