116 



GASTROPODA. 



meres has proceeded so far (Fig. 40 G, ent). According to Blochmann 

 (No. 7), the smaller entoderm-cells shift beneath the layer of ectoderm 

 towards the animal pole and here form, above the macromeres, a kind 

 of cap (Fig. 46). In this way an archenteron arises, which is bounded 



partly by smaller 



(All... 



t. 



Tim. 



Fig. 46. — Embryo of Neritina flvmatilis in optical 

 section (after Blochmann). bl, blastopore ; erf, ecto- 

 derm ; ent, entoderm ; mes, mesoderm. 



entoderm-cells and 

 partly by the mac- 

 romeres. Neritina 

 in this point more 

 nearly resembles the 

 forms considered 

 above, in which there 

 was a transition from 

 an epibolic to an in- 

 vagination - gastrula. 

 A cap of micromeres at 

 first lies on the large 

 macromeres, somewhat as in Fig. 40 F and G, but a cleavage-cavity 

 soon appears between the micromeres and the macromeres. As the 

 circumcrescence of the macromeres advances, the archenteron de- 

 velops, although in a way which deviates from that commonly met 

 with. 



In Urosalpinz, Fulgur, Purpura and Nassa also, gastrulation takes 

 place through epibole (Brooks, No. 17; McMurrich, No. 70; 

 Bobretzky, No. 11), and in these forms, on account of the great 

 abundance of yolk, other variations in the formation of the germ- 

 layers are caused. It has already been shown that in Nassa mutdbilu 

 the one of these forms which has received most attention, as well as 

 in Urosalpinx and Purpura, one of the macromeres which is specially 

 rich in yolk is far larger than the others (Fig. 42 D). The micro- 

 mere-layer lies on the macromeres in the form of a disc or cap (Fig. 

 42 E). When the micromeres grow out towards the vegetative pole, 

 the three smaller maci'omeres also take part in the process of shifting 

 and in so doing increase in number (Fig. 47 B, hi/). Finally, these 

 cell-complexes, which represent the rudiment of the entoderm, become 

 more and more shifted towards the vegetative pole (Fig. 47 C and 

 D). They line a cavity which corresponds to the future lumen of 

 the enteron. It is the protoplasmic parts of the macromeres that 

 are at first used for the formation of the epithelium of the enteron ; 

 the rest forms a kind of food -yolk upon which the cells of the germ- 

 layers lie like a germ-disc (Fig. 47 B). As far as can be seen from 



