COMPARISON OF THE GERMINAL LAYERS. 



299 



The above processes in the formation of the mesoblast are 

 for the most part easily explained by a comparison with the 

 lower types. The outgrowth of the mesoblast from the sides of 

 the primitive streak is a rudiment of the dorsal invagination of 

 hypoblast and mesoblast found in Amphibia ; and the apparent 



FIG. 188. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH AN EMBRYO 

 LIZARD TO SHEW THE RELATIONS OF THE NEURENTERIC CANAL (ne) AND OF 



THE PRIMITIVE STREAK (pr). 



am. amnion; ep. epiblast; hy. hypoblast; ch. notochord; //. body cavity; ne. 

 neurenteric canal ; pr. primitive streak. 



outgrowth of the mesoblast from the epiblast in the primitive 

 streak is no more to be taken as a proof of the epiblastic origin 

 of the mesoblast, than the continuity of the epiblast with the 

 invaginated hypoblast and mesoblast at the lips of the blasto- 

 pore in the Frog of the derivation of these layers from the 

 epiblast in this type. 



The division of the mesoblast into two plates along the dorsal 

 line of the embryo, and the formation of the notochord from the 



FIG. 189. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE 

 BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD, 

 HUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE. 



ep. epiblast; hy. hypoblast; ch. notochord; me. mesoblast; . nuclei in the 

 yolk of the germinal wall yk. 



axial hypoblast, are intelligible without further explanation. 

 The appearance of part of the mesoblast before the formation of 

 the primitive streak is a process of the same nature as the 



