W. K. BROOKS ON THE GENUS SALPA. 287 



Plate XXIV, Fig. 3, where they are marked by the letters Eckb, which 

 might be supposed to stand for "ectodermal blastomeres," if he did not 

 tell us explicitly on page 389 that the ectoderm of this species is derived 

 from the epithelial capsule (Epithelhugel). 



The ectodermal blastomeres seem to be more conspicuous in Salpa 

 fusiformis than in other species, for Salensky says, page 347, that 

 while the epithelial capsule (Ectodermkeim) is generally separated very 

 sharply from the embryonic cell mass, it is, at one end of the embryo, 

 so intimately related to the follicle cells (gonoblasts) that it is difficult 

 to determine the boundary between them ; and the blastomeres which lie 

 directly at this spot are covered only by the cells of the epithelial capsule 

 (Ectodermkeim). At a later stage, page 350, he says that the epithelial 

 capsule (Ectodermkeim) contains cells which differ greatly among them- 

 selves in both size and form. Some of these are similar in appearance 

 to the cells of the epithelial capsule as already described at an earlier 

 stage, and differ from them only in being flattened. " The others (Eckb) 

 are very much larger and very different in structure. They stain only 

 slightly in carmine, and contain a nucleus which is very similar to that of 

 the blastomeres. TJie appearance of these cells suggests that they are blasto- 

 meres which have passed out from the cell mass." 



Salensky believes that the ectoderm of Salpa democratica is derived 

 from the oviduct, and that in all other species it is derived from the 

 epithelial capsule, but I think all will agree that his position is untenable 

 until he has traced the history of the extra-follicular blastomeres, and 

 has proved that they take no part in its formation. 



I have shown, however, that they do give rise to the ectoderm, and 

 that the epithelial capsule is a transitory structure which is lost as the 

 ectoderm replaces it. 



SECTION 4. The Ectoderm of the Stolon and that of the Aggregated Salpa. 



All observers agree that the ectoderm of the stolon is derived directly 

 from the ectoderm of the embryo, substantially as I have described it on 

 page 68. 



In one minor point my own observations have shown that the older 

 accounts are inaccurate. It is usually stated that the ectoderm of the 

 embryo becomes pushed out into a tube by the growth of the other con- 

 stituents of the stolon, and Seeliger says (11), p. 13, that it is an evagi- 

 nation (Ausstulpung) from the ectodermal epithelium of the embryo. 



