SALPIDAE — -FOKMS WITH COVERING FOLDS. 



437 



In the next stages we timl ourselves on firmer ground (Fig. 218 A), 

 the most important organs having already developed. This stage is 

 characterised by the appearance of a cavity which, from its relations 

 to the rudiments of organs (corresponding with those described for 

 S. democratica-mucronata), we may regard as the body-cavity. Into 

 this cavity, a club-shaped mass of cells, the early rudiments of the 

 organs, hangs down from the upper surface <>f the embryo. The peri- 

 cardial rudiment (/»•) is, however, distinguished by being further 

 attached at its lower end. 



i 



a 



& m \ 



As 



Fig. 218. T\\ itogeuetic stages of Salpa pinnata (after Sai.kxskv). A, diagram- 

 matic median section i>t a younger stage combined from various figures by Salensky i 

 /!. older stage, b, blastomeres ; bk, " blood-forming bud " ; bl, blood-spaces in the 

 placenta ; d, atrial cavity ; d, enteric rudimeut ; dp, rout' of the placenta ; ec, ecto- 

 derm ; i . covering fold ; h, rudiment of In-art ; k, .nill ; m, muscle-hoops ; mp, 

 placenta] membrane ; n, rudiment of the uervous system ; //. placenta ; /<<•. peri- 

 cardial rudiment : ph, pharyngeal cavity. 



This cavity has been called by Salensky the secondary follicle- 

 cavity. Since, according to this author, only some of the rudiments 

 of the internal organs (the nervous system and the pericardial 

 rudiments) are formed from the inner cell-mass of the embryo, the 

 body-wall (together with the rudiment of the intestine) being 

 derived from the wall of the primary brood-sac ; this cavity has, for 

 Salensky, the same significance as the original cavity of the primary 

 brood-sac (Fig. 217,/). The latter, which is called by Salensky 

 the primary follicle-cavity, is said completely to disappear in those 

 obscure stages which lead up to the stage now being considered, and 



