CHAPTER V.

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.