220 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



to the construction of the embryo. So far as salpa is concerned, I assert 

 even more confidently than Seeliger that no definitive eggs ever give rise 

 directly, before fertilization, to follicle cells, and while certain of the eggs 

 of Salpa democratica unquestionably become abortive, there is no reason 

 to think they continue to live in another form or become converted into 

 other cells. I have shown in the chapter on embryology of Salpa pinnata 

 that this species, at least, conforms to the analogy of the rest of the animal 

 kingdom, and that its germ layers are derived from the blastomeres of 

 the fertilized egg, while the follicle cells take no part, except a nutritive 

 one, in the formation of the embryo, and lose their identity as living cells 

 before they contribute to its growth. They help to form the body, it is 

 true, but they have no more to do with its cellular constitution than the 

 radiolarians which pass into its stomach. 



While I am forced to oppose, in toto, Salensky's view of the fate of 

 the follicle cells, and am therefore unable to attach any weight to his 

 argument on this basis in favor of the homology of the follicle cells, I 

 am nevertheless disposed to accept his homology rather than Seeliger's, 

 and to regard the epithelial cells of the ovary and their derivatives, the 

 follicle and fertilizing duct, as homologous with germ cells. 



This view presents no inherent difficulty. The yolk gland of a 

 cestode or of a turbellarian unquestionably consists of cells homologous 

 with eggs, and there can be no doubt that if we could trace it back to its 

 origin in remote ancestors we should find its equivalent, not in any organ 

 of the body, but in germ cells. Specialization of ovarian ova for some 

 other purpose than sexual reproduction is not unusual, and the gas- 

 teropod molluscs and the insects present many examples. In most cases 

 the function is a nutritive one, and as I shall show that the function of 

 the follicle cells of salpa is exclusively nutritive, there is abundant 

 analogy for the view that they are homologous with eggs, nor is there 

 any inherent difficulty in believing that an accessory reproductive organ, 

 like the fertilizing duct, may have had a similar origin. 



Whether this is the case or not is a question of phylogeny to be 

 determined by the methods of comparative anatomy, and Salensky, 

 p. 75, appeals to Bolles Lee's account of the reproductive organs of 

 appendicularia and to the studies of the reproductive organs of ascidians 

 by E. van Beneden and Julin in support of his position, and certainly 

 the weight of evidence seems to favor the view that all the cells of the 

 germinal mass are germ cells, although it must be confessed that the 

 subject is hypothetical and that there is little basis for a decided opinion. 



