PYKOSO.MA DKVKLOI'MKNT OP THE I'YATHOZOOID. 



395 



[n the meantime the germ-disc has separated somewhat from the 

 surface of the food-yolk ( Fig. 188). The cavity thus formed is the enteric 

 cavity, which originally appears covered by the entoderm (en) only on 

 its upper surface. Ai a later stage the entoderm covers the whole of 



cl 



-71 



A. 



P c - 



V 



s 



> n 



-p 



\ 



. 





y""*-- en 



Fig. 189. Germ-disc of Pyrosoma with the atrial orifice developed (after KLowa- 

 lbvskt). cl, cloaca; en, endostyle ; n, aervous system; p, peribranchial tubes; 

 pc, pericardia] sac : pc', the posterior tubular eontinuatiou of the same. 



the cavity, its lateral edges bending downward and growing towards 

 one another (Kowalevsky). According to Salensky, the yolk-cells 

 also take part in this ventral closure of the enteric rudiment (Fig. 

 188, -/:) by coming to the surface of the food-yolk and changing into 



ma 



Fig. 190. Transverse section through the posterior region of a germ-disc at the stage 

 depicted in Fig. I s '.' (after Salensky). dh, enteric cavity ; ec, ectoderm ; en, ento- 

 derm ; es, rudimenl of the endostyle : his, mesoderm ; ///<. peribranchial tubes; pc, 

 pericardial tube. 



epithelial cells of the entoderm (p. 392). While the enteric rudiment 

 in this way becomes a tube closed on all sides (Fig. 190), a median 

 infolding, the rudiment of the endostylt (es) becomes visible in the 

 posterior half of its upper wall. 



