W. K. BROOKS ON THE GENUS SALPA. 271 



cant change takes place in the follicular wall. This change is the 

 growth (Wucherung) of the wall, which begins at the lower part of 

 the embryo, where the wall becomes greatly thickened and consists of 

 vacuolated tissue. The growth of the follicular wall is brought about by 

 the great multiplication of its cells, which are set free and lose their con- 

 nection with each other and change their form. These changes begin at 

 the bottom of the embryo and extend upwards. The section which is 

 represented in his Fig. 29. which corresponds to my Plate XIII, shows, in 

 the upper part of the embryo, the unmodified tissue of the follicular wall, 

 which consists of densely crowded polygonal cells. This tissue is found 

 in only a small part of the section, however, and he says that, in the 

 greater part, the follicle wall is greatly thickened and consists of an inter- 

 cellular fluid, and of cells which move freely through it. That these 

 cells are free and able to move is shown, as he points out, by their varia- 

 bility and their amoeboid shape. 



The outline of the somatic layer persists for a little while after the 

 disintegration begins, as my Plate XIII shows, but the whole structure 

 soon breaks down and disappears, and Salensky\s account, as well as my 

 own, shows that its identity as a tissue is quickly destroyed, although he 

 says, p. 159, that we cannot speak of the disappearance of the follicle, 

 since its cells proliferate and furnish the material for the formation of 

 the mesodermal and endodermal structures. 



He says, however, p. 129, that as the result of this growth the whole 

 of the secondary follicular cavity (body cavity) becomes filled with 

 wandering amoeboid cells from the walls of the follicle, and that there 

 is thus formed between the epithelial capsule (Ectodermkeim) and the 

 organs of the embryo a mass of wandering cells, which he regards as 

 equivalent to a mesoderm. 



His account of the phenomenon in Salpa fusiformis, p. 352, is very 

 similar, as he says that the cells of the peripheral layer of the follicle 

 separate from each other and become star-shaped, while a homogeneous 

 semi-fluid substance, which does not stain in carmine, appears between 

 them. 



It will thus be seen that his view of the fate of the somatic layer is 

 almost identical with the account which I have given on page 44, 

 although he believes that the fate of other follicular structures is very 

 different. 



