EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 235 



After the blastoderm has become two layered, and endoderm formation 

 is complete or nearly so, the invagination of mesoderm takes place 

 through the same blastopore. The tissues move to the blastopore, sink 

 down through it, and are pushed forward in the shape of a tube which 

 extends towards the anterior. This used to be known as the 'archenteric 

 canal', but since, according to the vital staining experiments of Pasteels, it 

 contains no endoderm but is made up wholly of mesoderm, it should, he 

 suggests, be called the 'chorda-mesodermal canal'. Its roof will eventually 

 become notochord, its sides somites, while its floor migrates further later- 

 ally to form the side-plates. At its anterior end the floor disappears at later 

 stages, so that the canal leads from the blastoporal opening right through 

 into the subgerminal cavity (Fig. 11. 6). 



Unfortunately, nothing whatever is known of the causal mechanisms 

 of early reptilian development. The very slow differentiation of the eggs 



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FlGURE 11.6 



Gastrulation in the turde Clemmys. A, map of presumptive areas, before the 

 invagination of the endoderm (shading as in Figure 11.3, dashed vertical 

 lines, extra-embryonic). B, movements of surface tissues tov^^ards the blasto- 

 pore, from which the chorda-mesoderm canal extends. C, formation of 

 endoderm from the blastopore at the beginning of gastrulation. D, the 

 movements of tissues at a later stage through the blastopore into the chorda- 

 mesoderm canal. (From data of Pasteels.) 



