294 



MESOBLAST AND NOTOCHORD. 



n.al 



posterior part of the archenteron is clearly comparable with the 

 analogous process in Amphioxus. 



Vertebrata. In turning from Amphioxus to the true 

 Vertebrata we find no form in which diverticula of the primi- 

 tive alimentary tract give rise to the mesoblast. There is 

 reason to think that the type 

 presented by the Elasmo- 

 branchii in the formation of 

 the mesoblast is as primitive 

 as that of any other group. 

 In this group the mesoblast 

 is formed, nearly coincidently 

 with the hypoblast of the 

 dorsal wall of the mesenteron, 

 as two lateral sheets, one on 

 each side of the middle line 

 (fig. 182 m). These two 

 sheets are at first solid 

 masses ; and their differen- 

 tiation commences in front 

 and is continued backwards. 

 After their formation the 



FlG. 182. TWO TRANSVERSE SECTIONS 

 OF AN EMBRYO PRISTIURUS OF THE SAME 

 AGE AS FIG. 17. 



A. Anterior section. 



B. Posterior section. 



mg. medullary groove ; ep. epiblast ; hy. 

 hypoblast ; n.al cells formed round the nuclei 

 of the yolk which have entered the hypo- 

 blast ; m. mesoblast. 



The sections shew the origin of the 

 mesoblast. 



notochord arises from the 

 axial portion of the hypo- 

 blast (which had no share in 

 giving rise to the two mesoblast plates) as a solid thickening 

 (fig. 183 ch') } which is separated from it as a circular rod. Its 

 differentiation, like that of the mesoblastic plates, commences in 

 front. The mesoblast plates subsequently become divided for 

 their whole length into two layers, between which a cavity is 

 developed (fig. 184). The dorsal parts of the plates become 

 divided by transverse partitions into somites, and these somites 

 with their contained cavities are next separated from the more 

 ventral parts of the plates (fig. 185 mp). In the somites the 

 cavities become eventually obliterated, and from their inner 

 sides plates of tissue for the vertebral bodies (fig. 186 Vr) are 

 separated ; while the outer parts, consisting of two sheets, 

 containing the remains of the original cavity, form the muscle- 

 plates (mp). 



