CHAPTER V.. Stages B-G, Pp. 298-314.

STAGES B-G, pp. 298-314.

General features of the epiblast.--Original uniform constitution. Separation into lateral and central portions. The medullary groove.--Its conversion into the medullary canal. The mesoblast.--Its division into somatic and splanchnic layers. Formation of protovertebræ. The lateral plates. The caudal swellings. The formation of the body-cavity in the head. The alimentary canal.--Its primitive constitution. The anus of Rusconi. Floor formed by yolk. Formation of cellular floor from cells formed around nuclei of the yolk. Communication behind of neural and alimentary canals. Its discovery by Kowalevsky. Its occurrence in other instances. General features of the hypoblast. The notochord.--Its formation as a median thickening of the hypoblast. Possible interpretations to be put on this. Its occurrence in other instances.