ix MOLLUSCA 311 



which span the blastocoele, extending from gut to ectoderm. In 

 A B 



coe 



coe 



FIG. 245. Formation of the coelom in. Pnhnlina rirlpara. (After Erlanger.) 



A, optical frontal section of embryo in the .stage when the coelom is being formed. B, sagittal 

 section of embryo in this stage. C, transverse section in this stage, coe, coelomic pouch ; </, gut ; 

 p.ti; prototroehal cells. 



the hinder end of each streak 

 there is, however, a compact 

 mass which becomes hollowed 

 out to form a coelomic vesicle, 

 the rudiment of one of the 

 pericardial sacs. The two nieso- 

 dermic bands then fuse together 

 in the middle line behind, and 

 the pericardial sacs become 

 pressed against one another so 

 that their conjoined walls form 

 a septum (sept, Fig. 247, B). 



Tonniges denies point-blank 

 the existence of this ventral 

 sac. According to him the 

 adult mesoderm arises as an 

 ectoderuial proliferation, on 

 each side of the middle line, 

 which gives rise to the 



i\ 11 



seen 



i. 246. Optical frontal section <>!' aii einl>ryo 

 of I'l/linU/iti 1-irijiiirn a little older tlian those 

 ivpresented in figure 245. (After Krlanger.) 



I. ii, larval ki'lnry ; m.b, nn'sublaxtir baml : 

 p.tr, prot'it rnc'lial c-i'lls. 



irregular mass of cells 



Erlanger. 



Now, \ve may quite 



