W. X. BROOKS ON THE GENUS SALPA. 219 



some of the cells which are thus formed become ova, and others follicle 

 cells. 



Seeliger says (11, p. 36) that he could not determine whether the cells 

 which are to become eggs are different from the others from the first or 

 not, but he thinks that this is not the case, and that it is their position 

 alone which determines that the central cells shall become eggs. I can- 

 not find the slightest difference between the peripheral cells and the 

 central ones before the differentiation takes place, and agree with 

 Seeliger that the part which falls to each cell at the time when the 

 differentiation takes place is determined by its position, although I 

 believe that all are potentially germ cells. 



I have never found any trace of multiplication in cells which have 

 become definite ova, until they produce true blastomeres by segmenta- 

 tion after fertilization, and salpa gives no support to the view that the 

 follicle cells are true definitive eggs at any stage of their ontogenetic 

 history, and I fully agree with Seeliger (11, p. 37) that there is no 

 evidence that the true eggs of salpa can revert to their embryonic 

 condition or become converted into any other sort of cells capable of 

 continuous life. 



Salensky seems to hold the same opinion, for he says (17, p. 80), 

 " I shall not here discuss the question of the origin of the test cells, since 

 this can be settled only by special investigation; so far as salpa and 

 pyrosoma are concerned, it may be stated with confidence that the 

 calymmocytes are extra ovular in origin and are simply migrating 

 follicle cells." 



There are no " test cells " as distinguished from follicle cells, and these 

 latter are certainly not derived from definitive eggs. The discussion of 

 the interminable " test cell " literature is unprofitable, but Seeliger says 

 that if we grant that in certain tunicates they are derived from the egg, 

 this would seem to indicate simply that the cell in question is not a true 

 egg, but rather an embryonic cell, consisting, in part, of somatic material 

 (11, p. 43). 



Salensky unhesitatingly adopts a view directly opposed to that of 

 Seeliger, and holds that the follicle is not part of the soma, but part of 

 the germ, and he says (17, p. 81) that the fundamental identity of the 

 egg cells and follicle cells is so evident that it is not necessary to dwell 

 upon it. He says that the correctness of this view is proved by the trans- 

 formation of true egg cells into follicle cells, and also by the fact that in 

 salpa and pyrosoma the follicle cells contribute, like ordinary blastomeres, 



