38 J. T. CUNNINGHAM. 
Fie. 27.—Diagram of the ancestral vertebrate gastrula in section, with 
elongated dorsal blastopore. 
Fic. 28.—Diagram of same gastrula when the closing of the blastopore 
from before backwards is almost complete. 
Fie. 29.—Diagram of section through plane of symmetry of Hlasmobranch 
ovum, at a stage in the growth of the blastoderm over the yolk before the 
formation of the neurenteric canal. 
Fig. 30.—Diagram of section through the plane of symmetry of Sauropsi- 
dan ovum, while the neurenteric canal is still open. 
Diagrams I, IJ, III, illustrating the morphology of the blastoderm in 
different classes of Vertebrates :—I. Condition of the blastoderm in Teleostei. 
II. In Elasmobranchii. IIL. In Sauropsida. 2x. Anterior end of embryonic 
rudiment. y. Posterior end of same, at which point concrescence of edges 
of primitive blastopore occurs. 4. Posterior end of primitive blastopore. 
The double line yz shows the extent of the rim at which inflection of 
hypoblast occurs. The single line at x indicates the internal limit of the 
dorsal hypoblast. 
