1 88 THE GERM-PLASM 



free-swimming animal. Some of the ectoderm cells of the part 

 in question then become transformed into egg- or sperm-cells. 



In the case of certain species of polypes, free sexual medusae 

 were produced in the earlier period of the development of the 

 species, but at the present day these do not become detached, 

 but always remain attached to the stock : they thus no longer 

 serve for the dispersal and ripening of the sexual products but 

 only for their production and ripening. These species illustrate 

 the different stages in the process of degeneration of the medusae 

 to mere gonophores, or sexual sacs. In some species the form 

 of the medusa is completely retained in the sexual persons of the 

 stock, only the eyes and marginal tentacles being absent ; in 

 others, the bell has become degenerated into a closed sac, the 

 walls of which still retain the circular and radial canals ; and in 

 other species again, these canals have also disappeared, only the 

 three characteristic layers of the medusa remaining, and even 

 these have become so thin that their presence can only be de- 

 tected in microscopic sections. Finally, these three layers also 

 undergo degeneration, the w^all then consisting of a single layer, 

 so that the derivation of the sac from the bell of the medusa can 

 only be proved indirectly. Throughout all these stages of de- 

 generation, however, the ova or spermatozoa always ripen in the 

 gonophores. 



The behaviour of the germ-cells is the chief point of interest 

 to us in the course of this process of degeneration. For the 

 entire degeneration of the medusa proceeds from its germ-cells, 

 and 'is due to the fact that t/ie development of the latter has 

 gradually to be thrown back to earlier stages, so that the sexual 

 elements are ripened more quickly. 



It will not be necessary to enter into the reasons for this 

 hastening of the sexual maturity ; it is sufficient to know that in 

 some species in which the medusae become detached, e.g., Podo- 

 coryne cornea., the egg-cells are developed earlier than the 

 medusae in which they subsequently ripen, and in proportion as 

 the degeneration of the medusa advances the place of origin 

 of the germ-cells recedes more and more into the older and 

 earlier formed parts of the stock. The advantage of this is, 

 that the germ-cells develop earlier, and afterwards enter the 

 germ-sacs in a riper stage : they thus reach maturity much more 

 quickly. 



The remarkable thing about this process is the fact that 



