COMPARISON OF THE GERMINAL LAYERS. 



277 



Here also the blastopore lies at the hinder end of the 

 medullary groove, and on the closure of the groove becomes 

 converted into a neurenteric passage. 



In the true Vertebrates the types which most approach 

 Amphioxus are the Amphibia, Acipenser and Petromyzon. 

 We may take the first of these as typical (though Petromyzon is 

 perhaps still more so) and fig. 170 A B C D represents four 

 diagrammatic longitudinal vertical sections through a form 



A C 



ep 



B 



D 



FIG. 170. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF 

 BOMBINATOR AT TWO STAGES, TO SHEW THE FORMATION OK THE GERMINAL LAYERS. 

 (Modified from GStte.) 



ep. epiblast ; m. dorsal mesoblast ; ;;/. ventral mesoblast ; hy. hypoblast ; 

 yk. yolk ; x. point of junction of the epiblast and hypoblast at the dorsal side of the 

 blastopore ; al. mesenteron ; sg. segmentation cavity. 



