THE DEVELOPMENT OF A MOLLUSK 47 



Immediately around the pore left by the closing of the edge 

 of the ectoblasts is seen on this model the depression which 

 indicates the beginning of the future mouth of the embryo. For 

 a short time the pore itself is closed, but soon opens again and 

 communication is thus established between the exterior and the 

 internal cavity of the embryo. The structure of the embryo at 

 this time may be represented diagrammatically as in Fig. e. 



MODEL 18 



The ectoblasts completely enclose the egg, leaving only a narrow pore 

 (blastopore), about which is seen a depression. The derivation of ectoblasts 

 from the three quartets indicated on the models by the coloring. The various 

 •regions of the future larva are becoming more sharply defined. 



From this time on, the development consists of the dififerentia- 

 tion by growth of the multiplying cells of these three separate 

 layers into the specialized organs of the body. 



The ectoderm cells which, as shown by the number of nuclei, 

 are already very numerous, multiply rapidly in certain areas 

 indicated by the slight outgrowths on the surface. These soon 

 become more pronounced and form the beginning of the ecto- 

 dermic organs of the embryo. 



[23] 



