W. K. BROOKS ON THE GENUS SALPA. 



61 



During the early stages of segmentation some of the cells which 

 surround the egg of Distaplia pass into the fissures between the blasto- 

 meres (p. 536) in much the same way that they do in Pyrosoma, but on a 

 much less extensive scale. 



Ultimately, after segmentation is well advanced, (p. 548), these cells 

 enter into the substance of the endodermal blastomeres, and Davidoff 

 has no doubt (p. 549) that they serve as food for the endoderm cells. 



COT G. 



CUT H. 



In cut G the outer follicle, 6, is separated from the surface of the 

 egg by an intervening space, which is filled with free follicle cells, 7, or, 

 as the author calls them, abortive eggs, which have migrated into it 

 from the capsule. As the blastomeres are marked off by segmentation 

 these cells wander into the spaces between them, where some of them are 

 shown at 5. Except for these spaces filled by migrating cells, the blasto- 

 meres are in contact, and there is no empty segmentation cavity. As 

 development goes on the migrating cells penetrate the substance of the 

 blastomeres, as shown in cut H, and are used up as food, so that at last 

 all traces of the follicle disappear. In Distaplia the blastomeres are so 

 much larger than the migratory cells, at the latest stage when these are 

 found, that the two sorts of cells are easily distinguishable, and it is 

 certain that the follicle cells take only a nutritive part in the construc- 

 tion of the embryo. 



. In Pyrosoma and in Salpa, most of the migratory cells retain their 

 individuality until the blastomeres have become so small by repeated 

 division that they are no larger than the follicle cells, and it becomes 

 very hard to follow the course of each cell and to trace its fate. I have 

 shown that while this is difficult in Salpa pinnata, it is not impossible, 

 and that the ultimate fate of all the follicle cells is to supply food for the 

 cells of the embryo. In the embryo of Pyrosoma, Salensky has traced 

 the history of the two sorts of cells until they are equal in size and no 



