234 



GASTEROPODA. 



lips of the blastopore. These cells form the hypoblast. The whole of the 

 protoplasmic matter of the yolk-cells is employed in the formation of the 

 hypoblast. The rest of them remains as a mass of yolk. A longitudinal 

 section of the embryo at a slightly later stage, when the blastopore has 

 become quite narrowed, is represented in fig. 105 C. The greater part of 

 the dorsal surface is not represented. 



Two definite organs have already become established. One of these is a 

 pit lined by thickened epiblast on the posterior and dorsal side (sg). This 



FIG. 105. LONGITUDINAL SECTION THROUGH THE EMBRYO OF NASSA MUTA- 

 BII.IS. (After Bobretzky.) 



A. Stage when the mesoblast is commencing to be formed. 



B. Stage when the yolk is half enclosed. The hypoblast is seen at the lips of the 

 blastopore. 



C. Stage when the blastopore (bp) is nearly obliterated. 



D. The blastopore is closed. 



ep. epiblast ; me. mesoblast ; hy. hypoblast ; bp. blastopore ; in. intestine ; 

 st. stomach ; /. foot ; sg. shell-gland ; m. mouth. 



is the shell-gland. The other is the foot (/) which arises as a ventral 

 prominence of thickened epiblast immediately behind the blastopore. The 

 hypoblast forms a ring of columnar cells round the blastopore. On the 



