422 



TUNICATA. 



completely filled by the embryo (Fig. 211 B). The wall of this 

 capsule, which is produced by the union of the oviduct and the 

 follicle is from this time called (although not very accurately) the 

 follicular epithelium. 



During the above changes, the shield-like thickening of the epi- 

 thelium round the aperture of the oviduct has risen up more and 

 more, and now forms a mound-like swelling (epithelial prominence, 

 Fig. 210, e, 211, a) projecting into the posterior portion of the atrial 

 cavity. As the oviduct continues to shorten, the follicle, with the 

 embryo, is brought into ever closer proximity to this prominence and 

 finally passes into it. At later stages, the prominence becomes con- 

 stricted at its base (Fig. 213), remaining connected with the wall of 

 the atrial cavity only by a thin stalk. The embryo now, enclosed in 

 a kind of brood-sac, projects into the interior of the atrial cavitv. 



FIG. 211. Stages iu the cleavage of Salpa democratica-mucronata (after SALBNSKY). 

 In A, the embryo undergoing cleavage still lies in the egg-follicle (c). In B, the 

 embryo has passed into the cavity of the oviduct (b). The follicular epithelium has 

 contracted (c) and represents the first rudiment of the placenta. C shows a later 

 stage, a, so-called epithelial prominence = outer lamella of the brood-sac ; b, 

 shortened oviduct = inner lamella of the brood-sac ; c, follicular epithelium = rudi- 

 ment of placenta ; /, blastomeres ; z, immigrated follicle-cells (not represented in 7> 

 and C*). 



The wall of this brood-sac, which, as we shall see, is merely pro- 

 visional, is double. The external wall (Fig. 211 C, a) is a modified 

 portion of the epithelium of the atrial cavity, and, in the inner wall 

 (Fig. 211 C, b) we recognise the follicular epithelium. It appears 

 that, in this stage, the opening of the oviduct into the atrial cavity 

 has completely closed. 



After cleavage has ended, the embryo forms a solid, rounded body 

 composed of numerous cells which, according to SALENSKY, are 

 mostly derived from the follicular epithelium, but, according to 

 TOD ABO, have been produced by fission from the blastomeres. We 

 shall for the present adopt the latter view as the more probable.* 



* [See editorial note, p. 421]. 



