280 



CKPHALOPODA. 



t^t- * 



l-'ii.. 131. — Diagrams illustrating tlie formation of the germ-layers. A, thickening of 

 the edge of the germ-disc ; B ami 0, differentiation of the yolk-epithelium. Further 

 extension of the germ-disc over the yolk. 1), differentiation of the rudiment of the 

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

 eiiteron; mes, mesoderm ; ect (in />), the rudiment of the cerebral ganglion. 



