244 



ME SO BLAST AND NOTOCHORD. 



The whole dorsal wall of the posterior part of the archenteron is 

 converted into the notochord (fig. 181 ch), and the lateral walls into 

 the mesoblast (me) ; so that the original lumen of the posterior part 

 of the archenteron ceases to be bounded by hypoblast cells, and dis- 

 appears as such. Part of the ventral wall remains as a solid cord of 

 cells {at'). The anterior part of the archenteron in front of the 

 notochord passes wholly into the permanent alimentary tract. 



The derivation of the mesoblast from the lateral walls of the 

 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 primitive alimentary 

 tract give rise to the mesoblast. There is reason to think that the 

 type presented by the Elasmobranchii in the formation of the meso- 

 blast is as primitive as that of any other group. In this group the 



mesoblast is formed, nearly coin- 

 cidently 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 notochord 

 arises from the axial portion of 

 the hypoblast (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, com- 

 mences in front. The mesoblast 

 plates subsequently become di- 

 vided for their whole length into 

 two layers, between which a 

 cavity is developed (fig. 184). 

 The dorsal parts of the plates 



Fig. 182. Two tkansveese sections of 



AN EMBRYO PbISTIUBUS 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 

 hypoblast; m. mesoblast. 



The sections shew the origin of the 

 mesoblast. 



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 (mjo). 



The undivided ventral portion gives rise to the general somatic 



