W. K. BROOKS ON THE GENUS SALPA. 45 



The next part of the follicle to disappear is the somatic lining of the 

 perithoracic system. As shown in Plate XLII, Figs. 6 and 7, the cells of 

 this layer become detached and acquire amoeboid outlines before the gill- 

 slits acquire their openings into the pharynx, and as I have already stated, 

 all of them ultimately separate and pass into the cavity of the cloaca, which 

 becomes so well filled with them that at the stage shown in Plate XVIII, 

 Fig. 8, the outline of the cloaca is hard to trace in the sections. The details 

 of the process of disintegration, which are shown in Plate XLII, Fig. 8, g", 

 are so much like the history of the somatic layer just described, that no 

 further account seems necessary. The disintegration of the visceral mass 

 of follicle cells begins with the formation of the cavity of the pharynx, as 

 already described and figured at c in Fig. 8. After the blastodermic epi- 

 thelium of the pharynx is formed, most of the visceral follicle cells are 

 left outside it in the body cavity, as shown at 8 in Plate XVII, Figs. 1, 2 

 and 3, where they form a secondary wall outside the blastodermic epithe- 

 lium, as they do also around the gill-slits, Plate XVI, Fig. 3, and the 

 cloaca, Fig. 2. All these cells ultimately degenerate and disappear, but 

 while most of them first become migratory and amoeboid, as shown at 8 

 in Fig. 9 of Plate XLII, others become vacuolated and break down and 

 disappear without any migration, as for example those which are shown 

 in Plate XVII, Fig. 6, included between the epithelium of the cloaca, g'" 

 and the ectoderm a, at the point where the cloacal aperture is to be 

 formed. The history of the process of degeneration is as follows : The 

 large irregular amoeboid follicle cells, usually with two nuclei, Plate XX, 

 Fig. 5, wander into all parts of the body cavity and become very much 

 vacuolated, and while this is going on the small amoeboid mesoderm cells 

 of the embryo lodge upon their surfaces, as they do upon the surfaces of 

 the embryonic cells, and give rise to a fibrous capsule around each follicle 

 cell or group of cells, as is shown in Plate XX, Figs. 5 and 6, Plate XXI, 

 Figs. 1 and 2, and Plate XIX, Figs. 9 and 10. Mesoderm cells or blood 

 corpuscles are often found also in the vacuoles of degenerating follicle 

 cells, as in Plate XX, Fig. 5. As the cells disappear they leave behind 

 them an empty meshwork of fibers, as is shown in Plate XXI, Fig. 2. 



While it is of course impossible to trace the history of each follicle 

 cell, I think there is sufficient evidence to warrant the statement that, 

 while the organization of the embryo is at first blocked out in follicle 

 cells, all of this scaffolding is afterwards torn down ; for I have found 

 follicle cells in the act of detachment in all the regions of the embryo and 

 in all the f ollicular structures. 



