206 



GASTROPODA. 



posterior outgrowth (Fig. 92, vl, hi), while the stomach {mg) arises 

 from a small dorsal part of the enteron lying between these two. 

 Into the stomach open the oesophagus, the intestine and the two 

 hepatic sacs. 



In the cases so far considered, the enteron has at first a sac-like 

 form : this, however, soon becomes differentiated by the concentration 

 of the nutritive yolk or by the absorption of albumen by the cells in 

 one part of the enteron. In other cases, however, the accumulation 

 of food-yolk in the entoderm is so great that the sac-like rudiment of 

 the enteron is not able to develop at once. In Fusus, for instance, 

 according to Bobretzky, at a time when the oesophagus, the shell- 

 gland and the mesoderm are already well developed, the entoderm 

 consists of only a few large cells which are to a great extent filled 



a. 



3$. 



fi G- 94. — Two transverse sections of an embryo of Fusus, A, through the foot; />, a 

 more posterior section (after Bobretzky). d, yolk ; ect, ectoderm ; /, foot ; I, liver ; 

 mil, entoderm lining the stomach ; mes, mesoderm ; <>t, otocyst ; pg, pedal ganglion ; 

 ■.. sub-velar cells. 



with yolk, having a small protoplasmic portion directed towards the 

 mouth (Fig. 93 A). At this point, the division of the macromeres 

 gives rise to new entoderm-cells which are much smaller and soon 

 rise up from the macromeres, tints forming the rudiment of the 

 midgut, especially that of the stomach, which then, through the 

 formation of a posterior conical process, gives rise to the intestine 

 (Fig. 93 A and B, md). The increase in number of the entoderm- 

 cells is continued at the expense of the food-yolk, which is now 

 pressed further back. While, ventrally, the stomach becomes more 

 distinctly marked oft' (Fig. 62, p. 151 and 93, mg), the recently 

 developed dorsal parts of the entoderm become filled with deuto- 

 lecithal spherules and thus have a glassy appearance like the 



