ix MOLLUSC A 311 



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

 A B 



coe 



cce 



FIG. 245. -Formation of the coelom in Paludina vivipara. (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 ; g, gut ; 

 p.tr, prototrochal 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 rneso- 

 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 



FIG. 246. Optical frontal section of an embryo 

 otPalvdina vivipara a little older than those 

 represented in figure 245. (After Erlanger.) 



adult mesoderm arises as an 

 ectodermal proliferation, on 

 each side of the middle line, 

 which gives JCISQ to the 

 irregular mass of cells seen by Erlanger. Now, we may quite 



l.n, larval kidney ; m.b, mesoblastie band ; 

 p.tr, prototrochal cells. 



