280 



CEPHALOPODA. 



J3. 



de 



FIG. 131. Diagrams illustrating the formation of the germ-layers. A, thickening of 

 the edge of the germ-disc ; B and C, differentiation of the yolk-epithelium. Further 

 extension of the germ-disc over the yolk, /^differentiation of the rudiment of the 

 enteron and the mesoderm ; de, yolk-epithelium ; ect, ectoderm ; md, rudiment of 

 enteron ; tiws, mesoderm ; ect (in D), the rudiment of the cerebral ganglion. 



