OF THE GERMINAL LAYERS IN GASïOROPODA. OO 



his change is, as Wilson remarks, after the entoiiiere-cell 

 differentiates itself from the left posterior ento-mesomere-cell, and 

 takes its seat along the median axis. In other words, the bilateral 

 symmetry appears just after the cells of the three germ-layers 

 are distinguished. This corresponds with the stage when the sixth 

 or seventh quartet of ectomeres is formed and the egg is com- 

 posed of 29-33 cells as is shown in the tabulated form below. 

 The exact period of the occurrence could not be expressed as it 

 varies with the species, and even in the same species with the 

 environment. 



4. The cleavage of the mesomere-cell takes place after the 

 bilateral symmetry is completely established. It is then subdivided 

 into two equal halves, which stand in a strict bilateral symmetry 

 with regard to the median axis. Shortly afterwards from each of 

 these cells mesoderm- cells are propagated, which by the further 

 division eventually form a layer. Hence, it is clear in these 

 species that the mesoderm-cells take their first origin from the 

 posterior quadrants of the blastomere as in many other species. 

 The differentiation of the three germ-layers strictly speaking 

 dates from the formation of the mesoderm-cells. It is at about 

 the stage of the eighth generation of ectomere-cells in Siphoîiaria 

 and of about the tenth generation in Ajjiysia. It is therefore 

 the 42-cells stage in the former, while it is the 50-cells stage in 

 the latter. Thus, the differentiation of the mesoderm-cells varies 

 in time not only in the species named above, but also in all 

 those forms before thoroughly investigated. The results of the 

 present investigation as well as of those of other authors are 

 compiled below^ in the form of tables to facilitate a comparative 

 study of the Molluscan development : — 



