ON THE STAGES OF EVOLUTION 223 



mosis at their anterior extremities through the junction 

 of the buccal diverticulum with the infundibulum in 



FIG. 84. DIAGRAMMATIC LONGITUDINAL SECTIONS OF ELASMOBRANCH 



EMBRYO AND BLASTODERM. (From Balfour.) 



A, younger stage with two primary layers ; B, more advanced stage with three layers 

 and invagination at the hinder end of the embryo ; C, still more advanced ; the 

 embryo raised from the blastoderm with neural and primitive alimentary canals 

 and neurenteric communication between them. 



ep> epiblast ; m, mesoblast ; x, epiblast continuous with hypoblast ; nc, neural canal ; 

 ch, notochord ; al, alimentary cavity ; sg, segmentation cavity ; 11, nuclei of the 

 yolk. 



the matrix of the hypophysis, the anterior and posterior, 

 lobes of which uniting elements respectively, constitute 



FIG. 85. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF 

 LACERTA. (From Balfour.) 



//, body cavity ; am, amnion fold ; nc, neurenteric canal ; ch, notochord ; hy, hypo- 

 blast ; ep, epiblast of the medullary plate ; pr, primitive streak. In the primitive 

 streak all the layers are partially fused. 



the developmental basis, the processes, of posterior neur- 

 enteric differentiation, and anterior neurenteric union, 



