SALPIDAE FORMS WITH COVERING FOLDS. 



437 



Iii the next stages we find 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 

 N. democratica-mveronata), 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 of the embryo. The peri- 

 cardial rudiment (pc) is, however, distinguished by being further 

 attached at its lower end. 



KM. 21.S. Two onto-enetic stages of ,sW//r/ />i,nita (after SALENSKY). A, diagram- 

 matic median section of a younger stage combined from various figures by SALENSKY I 

 />'. older >t;i,-v. li, lilastomeres ; bk, " blood-forming bud " ; bl, blood-spaces in the 

 placenta ; '/. atrial cavity ; '/. enteric rudiment ; <tp, roof of the placenta; ec, ecto- 

 derm; /. covering fold; h } rudiment of heart; k, gill; /, muscle-hoops ; mp, 

 placental membrane ; //. rudiment of the nervous system ; p, placenta ; pc, peri- 

 cftrdial rudiment : />h. pharyngeal cavity. 



This cavity has been called by SALENSKY' the secondary follicle- 

 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 

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

 1. rood-sac (Fig. 217, /). The latter, which is called by SALENSKY 

 th- /H-i/iHii-t/ follicle-cavity, is said completely to disappear in those 

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



