PYROSOMA DEVELOPMENT OF THE CYATHOZOOID. 395 



.In the meantime the germ-disc has separated somewhat from the 

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

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

 its upper surface. At a later staut- the entoderm covers the whole of 





FIG. 189. (Jerm-disc of /'i/roxo/tui, with the atrial orifice developed (after KOWA- 

 LEVSKY). d, cloaca ; >'n, endostyle ; n, nervous system ; p, peribrauchial tubes ; 

 l><\ pericardial sac ; /><', the posterior tubular continuation 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, dz) by coming to the surface of the food-yolk and changing into 



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

 depicted in Fig. 18P (alter SALKNSKY). dh, enteric cavity ; ec, ectoderm ; en, ento- 

 derm ; , rudiment of the endostyle : ///*. mesoderm ; pb, peribranchial tubes ; //'. 

 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 endontr//'- (<<) becomes visible in the 

 posterior half of its upper wall. 



