114 



GASTROPODA. 



macromeres and consequently appears much thickened. After the 

 mesoderm has become differentiated, the entomeres begin to increase 

 in number (Fig. 40 H, ent), and the whole entoderm becomes in- 

 vaginated into the cleavage-cavity, and thus a typical invagination- 

 gastrula forms (Planorbis, EABL). In Patella, on the contrary, an 

 extremely large solid ingrowth of macromeres takes place from the 

 vegetative pole of the blastula (Figs. 49 and 50, p. 124). From this 

 ingrowth, the mesoderm and entoderm become differentiated and, 

 at a later period, an archenteric cavity forms within the till now solid 

 entoderm (PATTEN, No. 83). 



In a few Gastropods, such as Bythinia and Limnaea, a cleavage- 

 cavity is present at an early stage, but this soon disappears ; the 



Tft. - 



FIG. 44. A-C, embryos of Firoloida Desmaresti in the stage of gastrula- formation 

 (after FOL). M, blastopore ; ect, ectoderm ; rk, polar bodies. 



blastula now becomes flattened, the macromeres prepare to invaginate, 

 and the micromeres, advancing towards the vegetative pole, grow over 

 the mesoderm which, has already formed, and a part of the entoderm 

 (BAY LANKESTER, No. 63; WOLFSON, No. 131; ERLANGER, No. 

 28). Gastrulation follows the same course in Paludina, with the 

 distinction that, in this form, the cleavage-cavity is from the first 

 very small, and the mesoderm only later becomes recognisable 

 (cf. p. 134, BiJTSCHLi, No. 18). In the Heteropoda also (Firoloida 

 and Carinaria) a more or less flattened blastula with a slit-like 

 blastocoele, the animal end of which is composed of small and the 

 vegetative of large cells (Fig. 44 A), gives rise by a similar process 



