26 



LAMELLIBBANCHIA. 



FIG. 12. A-C, embryos of Teredo during the for- 

 mation of the germ-layers (after HATSCHEK). The 

 entoderm-cells are lightly dotted, while th e mesoderm- 

 cells are more darkly marked ; the unshaded part is 

 ectoderm. 



Ostrea virginiana, it is altogether wanting (Figs. 12 and 14 A). The 

 micromeres then lie immediately upon the macromeres, the con- 

 sequence being that, as they multiply, they surround the latter. 



An epibolic gastrula is 



A B. _ 



12 and 14 A), such as, 

 according to LOVEN, 

 is found in Modiolaria 

 and Cardium. In the 

 last stages of cleav- 

 age, the two primary 

 germ- layers are 

 already differentiated, 

 the micromeres corre- 

 sponding to the ectoderm and the macromeres to the entoderm. 

 This also applies to those cases in which a cleavage-cavity forms and 

 the gastrula arises through imagination (fresh-water Lamellibranchs, 

 BAY LANKESTEB, ZIEGLEB, LILLIE). In Cyclon, for instance, 



a shallow depres- 



A sion forms in the 



blastula (Fig. 13 

 A\ the vegeta- 

 tive pole of which 

 is no longer to be 

 distinguished by 

 the larger size of 

 its cells, and, by a 

 further invagina- 

 tion of the cells at 

 this point, a small 

 archenteron is 

 formed (Fig. 13 

 B). This is also 

 the case in Pisi- 

 dium. The bias- 

 topore takes the 

 form of a slit lying 

 in the median line, 



and in this way the embryo early assumes a bilateral symmetry. 

 The blastopore soon closes, so that the archenteron loses its 

 connection with the exterior and lies as a closed sac in contact 



FIG. 13. A-C, sections through embryos of Cyclas cornea, A, 

 blastula-stage, B, gastrula-stage, G, stage succeeding the 

 closure of the blastopore (after ZIEGLER). bl, blastopore ; 

 ent, entoderm ; m, mesoderm ; oes, rudiment of the stomo- 

 daeum. 



