298 MESOBLAST AND NOTOCHORD. 



in the other forms. It arises, in fact, as in Elasmobranchii, as 

 two independent plates. The fact of these plates originating 

 from an invaginated layer can only be regarded in the light of 

 an approximation to the primitive type found in Amphioxus. 



In Petromyzon and the Newt the whole axial plate of dorsal 

 hypoblast becomes separated off from the rest of the hypoblast 

 as the notochord, and this mode of origin for the notochord 

 resembles more closely that in Amphioxus than the mode of 

 origin in Elasmobranchii. 



In Teleostei, there is reason to think that the processes in the 

 formation of the mesoblast accord closely with what has been 

 described as typical for the Ichthyopsida, but there are still 

 some points involved in obscurity. 



Leaving the Ichthyopsida, we may pass to the consideration 

 of the Sauropsida and Mammalia. In both of these types there 

 is evidence to shew that a part of the mesoblast is formed in situ 

 at the same time as the hypoblast, from the lower strata of 

 segmentation spheres. This mesoblast is absent in the front 

 part of the area pellucida, and on the formation of the primitive 

 streak (blastopore), an outgrowth of mesoblast arises from it as 



FIG. 187. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. 

 ep. epiblast; me. mesoblast; hy. hypoblast; mg. medullary groove. 



in Amphibia, etc. From this region the mesoblast spreads as a 

 continuous sheet to the sides and posterior part of the blasto- 

 derm. In the region of the embryo, its exact behaviour has not 

 in some cases been quite satisfactorily made out. There are 

 reasons for thinking that it appears as two sheets not united in 

 the axial line in both Lacertilia (fig. 126) and Mammalia (fig. 

 187), and this to some extent holds true for Aves (vide p. 156). 

 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 cJi) process. 



