COMPARISON OF THE GERMINAL LAYERS. 247 



volutions from the exterior; in Osseous Fishes, however, as solid 

 involutions, in which a cavity is secondarily established. 



There are strong grounds for thinking that in all Vertebrates the 

 mesoblast plates on each side of the notochord originate independ- 

 ently, much as in Elasmobranchii, and that the notochord is derived 

 from the axial hypoblast ; but there are some difficulties in the appli- 

 cation of this general statement to all cases. In Amphibia, Ganoids, 

 and Petromyzon, where the dorsal hypoblast is formed by a process 

 of invagination as in Amphioxus, the dorsal mesoblast also owes its 

 origin to this invagination, in that the indifferent invaginated layer 

 becomes divided into hypoblast and mesoblast. Amongst these forms 

 the mesoblast sheet, when separated from the hypoblast, is certainly 

 not continuous across the middle line in Petromyzon (Calberla) and 

 the Newt (Scott and Osborn), and doubtfully so 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 de- 

 scribed 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 evi- 

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

 the same time as the hypoblast, from the lower strata of segmen- 

 tation spheres. This mesoblast is absent in the front part of the 

 area pellucida, and on the formation of the primitive streak (blasto- 

 pore), an outgrowth of mesoblast arises from it as in Amphibia, etc. 

 From this region the mesoblast spreads as a continuous sheet to the 

 sides and posterior part of the blastoderm. In the region of the embryo, 



•y 



Fig. 187. Teansvebse section through an embryo Babbit of eight days. 

 ej). epiblast; me. mesoblast; hy. hypoblast; mg. medullary groove. 



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. 180). 



