MOLLUSC A. 



193 



example of a mai'ine Gasteropocl with a large food-yolk. The segmentation 

 has already been described, p. 83. It will be convenient to take up the de- 

 velopment at a late stage of the segmentation. The embyro is then formed 

 of a cap of small cells which may be spoken of as the blastoderm resting 

 upon four large yolk-cells of which one is considerably larger than the 



FlG. 105. LONGITUDINAL SECTION THBOUGH THE EMBRYO OF NASSA MUTABILIS. 



(After Bobretzky). 



A. Stage when the mesoblast is commencing to be formed. 



B. Stage when the yolk is half enclosed. The hypoblast is seen at the lips of the 

 blastopore. 



C. Stage when the blastopore (bp) is nearly obliterated. 



D. The blastopore is closed. 



(p. epiblast; me. mesoblast; hy. hypoblast; bp. blastopore; in. intestine; 

 Kt. stomach; /. foot; sg. shell-gland; m. mouth. 



others (fig. 104 A). The small and the large cells are separated by a 

 segmentation cavity. The general features at this stage are shewn in 

 fig. 105 A, representing a longitudinal section through the largest yolk- 

 cell and a smaller yolk-cell opposite to it. The blastoderm is for the most 

 part one cell thick, but it will be noticed that, at the edge of the 

 blastoderm adjoining the largest yolk-cell, there are placed two cells under- 

 neath the edge of the blastoderm (me). These cells are the commencement 

 of the mesoblast. In the later stages of development the blastoderm con- 

 tinues to grow over the yolk-cells, and as it grows the three smaller yolk- 

 cells travel round the side of the largest yolk-cell with it. As they do 

 so they give rise to a layer of protoplasmic cells (fig. 105, //.;/) which form 

 a thickened layer at the edge of the blastoderm and therefore round the 



B.E. 13 



