118 GASTROPODA. 



poda (Clione, KNIPOWITSCH, No. 55). In this case also, the division 

 of one of the four macromeres is said to give rise to two cells which 

 are soon driven inwards, these two symmetrically placed cells denot- 

 ing the posterior end. KNIPOWITSCH conjectures that in those 

 Pteropoda in which, according to FOL, one of the macromeres is 

 distinctly smaller than the others, this smaller macromere yields the 

 primitive mesoderm- cells (Fig. 41 A, III). In Clione, each of the 

 two cells which arise by the division of the macromere again divides 

 into two large cells (mesoblasts, Fig. 45 B, mes), which now take up 

 a symmetrical and bilateral position at the posterior end and, by 

 continuous multiplication, give rise to smaller cells. 



The radial character of the cleavage, which is so marked during the 

 early stages (Fig. 40 C-E), is much modified by the differentiation 

 of the mesoderm, and, when the two mesoderm-cells appear, the germ 

 attains a true bilateral symmetry (Fig. 40 H). This is the case 

 in Planorbis and a similar condition is shown in Bythinia also. As 

 in Planorbis, the primitive mesoderm-cells in Bythinia arise from one 

 of the posterior blastomeres, which is to be regarded as a mesentomere, 

 i.e., it divides into two cells, one of which remains as an entomere 

 in the position occupied by the posterior macromere, while the other 

 shifts slightly forward. This latter cell divides into two cells in 

 such a way that the two lie side by side ; these are the mesodermal 

 teloblasts which give rise to the mesoderm-bands (v. ERLANGER, No. 

 28). The mesoderm rises in a similar manner in Crepidula (CONKLIN, 

 No. 24) * and Neritina (BLOCHMANN, No. 7), although a few slight 

 modifications are here brought about by gastrulation taking place 

 through epibole in consequence of the large size of the macromeres, 

 or by a near approach to this form of gastrulation. In Neritina, 

 a cell becomes detached from one of the posterior macromeres which, 

 by division, gives rise to the two mesoderm-cells (Fig. 40 6?). This 

 process can be made out very distinctly in the eggs of an Opistho- 

 branch (Umbrella) examined by HEYMONS (Fig. 48). Here also a 

 smaller entoderm-cell and a larger mesoderm-cell (B, ent and m) 

 arise through the division of one of the posterior macromeres (A and 

 B}. This primitive mesomere divides into two laterally placed meso- 

 derm-cells (C, um) which soon give rise to the two mesoderm-bands, 

 formed of a few large cells containing yolk and other smaller cells 

 (D and E). The rise of the mesoderm from one of the posterior 



* [CONKLIN (No. IV.) now finds that, in Crepidula, the mesoderm does not 

 arise until after two further divisions, but regards this as an exceptional con- 

 dition. ED.] 



