194 



GASTEROPODA. 



lips of the blastopore. These cells form the hypoblasfc. 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 

 seqtion 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 

 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 

 posterior side its cells have bent over so as to form a narrow tube (in), the 

 rudiment of the intestine. 



In the next stage (fig. 105 D) the blastopore completely closes, but its 

 position is marked by a shallow pit (ni) where the stomodfeum is eventually 

 formed. The foot (f) is more prominent, and on its hinder border is 

 formed the operculum. The shell-gland (not shewn in the figure) has 

 flattened out, and its thickened borders commence to extend especially over 

 the dorsal side of the embryo. A delicate shell has become formed. In 

 front of and dorsal to the mouth, a ciliated ring-shaped ridge of cells, which 

 is however incomplete dorsally, gives rise to the velum. On each side of the 

 foot there appears a protuberance of epiblast cells, which forms a provisional 

 renal organ. The hypoblast now forms a complete layer ventrally, bound- 

 ing a cavity which may be conveniently spoken of as the stomach (st), 

 which is open to the yolk above. Posteriorly however a completely closed 

 intestine is present, which ends blindly behind (in). 



The shell and with it the mantle grow rapidly, and the primitive 

 symmetry is early interfered with by the shell extending much more 

 towards the left than the right. The anus soon becomes formed and places 

 the intestine in communication with the exterior. 



With the growth of the shell and mantle the foot and the head become 



sharply separated from the visceral 

 sack (fig. 106). The oesophagus (m.) 

 becomes elongated. The eyes and 

 auditory sacks become formed. 



With further growth the asym- 

 metry of the embryo becomes more 

 marked. The intestine takes a trans- 

 verse direction to the right side of 

 the body, and the anus opens on the 

 right side and close to the foot in the 

 mantle cavity which is formed by an 

 epiblastic invagination in this region. 

 The cavity of the stomach (fig. 106, 

 st) increases enormously and passes 

 to the left side of the body, pushing 

 the food-yolk at the same time to 

 the right side, and the point where 

 it communicates with the intestine 

 becomes carried towards the posterior dorsal end of the visceral sack. 

 The walls of the stomach gradually extend so as to narrow the opening 



FIG. 106. LONGITUDINAL SECTION 

 THROUGH AN ADVANCED EMBRYO OF NASSA 

 MUTABILIS. (After Bobretzky.) 



/. foot; m. mouth; ce.v. cephalic 

 vesicle; st. stomach. 



