248 



MESO BLAST AND NOTOCIIORD. 



In Lacertilia (fig. 188) and Mammalia, the axial hypoblast becomes 

 wholly converted into the notochord, which at the posterior end of 

 the body is continued into the epiblast at the dorsal lip of the 

 blastopore ; while in Birds the notochord is formed by a very similar 

 (fig. 189 ch) process. 



Fig. 188. Diagrammatic lonoitudinal section through an embryo Lizard to shew 

 the relations of the neurenteric canal {lie) and of the primitive streak (jpf). 

 am. amnion; ep. epiblast; hy: hypoblast; ch, notochord; pp. body cavity; ne, 

 neurenteric canal; pr. primitive streak. 



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 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 meso- 



FiG. 189. Transverse section through the embryonic region of the blastoderm 

 OF A Chick at the time of the formation of the notochord, but before the 

 appearance of the medullary groove. 

 ep. epiblast; Jiy. hypoblast; ch. notochord; me. mesoblast; n. nuclei in the yolk 



of the germinal wall yk. 



blast at the lips of the blastopore 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 

 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 differenti- 

 ation of hypoblast and mesoblast in Elasmobranchii without an 

 invagination. 



