CHAPTER IV 



THE SEGREGATION OF THE GERM CELLS IN 

 PORIFERA, CCELENTERATA, AND VERTEBRATA 



The history of the germ cells has not been seriously 

 investigated in a number of groups of animals, but, as 

 will be demonstrated in Chapters V and VI, there 

 are many species belonging to widely separated 

 groups in the animal series in which the germ-cell 

 cycle is almost as well known as in Miastor. On 

 the other hand, the three phyla to be discussed in 

 this chapter have been carefully studied for many 

 years, but an early segregation of germ cells has not 

 yet been established in them to the satisfaction of a 

 majority of investigators. It seems strange because 

 of the uncertainty of the morphological continuity 

 of the germ cells in these animals that one of these 

 groups, the Cgelenterata, should have furnished 

 the material upon which Weismann based his elabo- 

 ration of the germ-plasm theory. 



1. PORIFERA 



Sponges reproduce asexually by budding and by 

 the formation of gemmules, and sexually by means of 

 ova and spermatozoa. Budding occurs in almost all 

 sponges. In most cases the buds remain attached 

 to the parent (continuous budding) ; but in some 



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