REPRODUCTION 



75 



the similarities seem more significant than the differences. The point of 

 unmistakable correspondence is to be seen in the important part played 

 by the proliferation zone of the blastoporal rim or "germ ring." The 

 amphibian gives little evidence of anything comparable to the paired 

 mesodermal pouches which push out from the dorso-lateral endoderm of 

 Amphioxus. However, about three quarters of the ultimate number of 

 mesodermal segments in Amphioxus have origin in the blastoporal ring. 

 In amphibians the germ ring is the all-important source of mesoderm. 



During the process of gastrulation in the amphibian the material 

 destined to become mesoderm lies within the advancing edge of the over- 

 growing fold (Fig. 44) which is the chief agency in the enclosing of the 

 yolk. As the edge of this fold, the narrowing blastoporal rim, advances, 



NP\ ,NC 



Fig. 48. — Sections of an amphibian embryo at an early stage in the development 

 of the notochord and mesoderm. Semi-diagrammatic. A , median longitudinal section. 

 B, transverse section near the middle of the longitudinal axis. .4, archenteron; BP, 

 blastopore; EC, ectoderm; EN, endoderm; MES, mesoderm; NC, notochord; NP, neural 

 plate. 



it (in effect) leaves behind it — "behind" being anterior because the fold 

 advances posteriorly — a trail of potential mesoderm which, however, is 

 at first in no way distinguishable from other material destined to be 

 permanently endoderm (Fig. 44C', EN). That is, the two materials 

 together and in no way delimited from one another constitute the deeper 

 layer of the overgrowing fold. Later this layer virtually splits (the process 

 called delamination) to form two layers, an inner one abutting on the 

 archenteric cavity and an outer one which is then recognizable as a definite 

 mesoderm (Fig. 485). This layer, although now distinct from the endo- 

 derm which parallels it, for a time retains continuity with its source, the 

 proliferation zone about the blastopore (Fig. 48.4). Initiated in this way, 

 the mesoderm extends into the lateral and anterior regions of the embryo 

 partly by growth within itself, partly by continued contributions from 

 the blastoporal growth zone and possibly augmented by the detachment 

 of cells from neighboring surfaces of the endoderm. 



