26o COMPARATIVE ANATOMY 



cells lying immediately under the neural groove ; so there 

 is a stage, represented in transverse section in fig. 6i, A, 

 in which the hypoblast and mesoblast are fused in the mid- 

 dorsal line, but are separated laterally where they form the 

 roof and walls of the enteron. The floor of the enteron 

 is formed by the yolk-cells, into which the hypoblast passes 

 insensibly, but the mesoblast continues to spread round the 

 embryo between the epiblast and the yolk till it completely 

 encloses the latter. The ventral extension of the mesoblast 

 differs in different regions of the embryo. It is obvious, 

 from an examination of fig. 59, G, that in the neighbourhood 

 of the blastopore — that is, at the hinder end of the embryo — 

 the mesoblast forms a complete investment to the yolk, for 

 it grows inwards all round the lips of the blastopore. Further 

 forward the mesoblast extends some way down on either side 

 of the yolk-cells, and at the extreme anterior end it forms only 

 the roof and side walls of the enteron, and does not extend 

 down the sides of the yolk. At the stage when the neural 

 folds have become distinct, the median tract of cells, which 

 in the preceding stage was continuous with the mesoblast 

 sheets at the side and the hypoblast below, becomes separated 

 from both layers and forms a solid rod of cells, the notochord, 

 lying beneath the neural groove. The notochord is first 

 differentiated in the hinder half of the embryo, and extends 

 gradually forward. Posteriorly it does not reach the blastopore, 

 but ends in a mass of cells lying in front of the dorsal lip, 

 in which mesoblast and hypoblast are indistinguishably blended 

 together. In the frog the notochord appears to be formed 

 from a tract of cells in which mesoblast and hypoblast are 

 blended together, and at the time of its appearance as a 

 separate structure it has the appearance of being formed 

 as a differentiation of the median tract of the mesoblast. 

 In the newt, and some other long-tailed Amphibia (e.g. the 

 Axolotl), the notochord is distinctly formed from a groove 

 in the dorsal median line of the hypoblast, which is pinched 

 off and converted into a solid rod, just as is the case in 

 Amphioxus. In the frog the origin of the notochord from 

 the hypoblast is obscured. The hypoblast and mesoblast 

 are first differentiated out of the lower layer cells, and then 

 the notochord separates out from th'fe mesoblast, and its 

 ancestral origin from hypoblast would not be recognised if 



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