REPRODUCTION. 239 



area, Hensen's node, that represents the beginning invagination of the ecto- 

 derm in the formation of the embryonic mesoderm and notochord to be con- 

 sidered later. That portion of the embryonic shield that gives rise to the 

 embryo itself becomes distinctly outlined laterally and in the head and tail 

 regions of the neural groove. Just external to this area, the embryonic area 

 proper, is a transparent area, the area pellucida, beyond which is the area 

 opaca in which the first blood-vessels appear. 



Mesoderm and Notochord. So far in the embryonic area only ectoderm 

 and entoderm exist. Hensen's node, at the head end of the primitive streak 

 represents an invagination (gastrulation) of ectoderm between ectoderm and 

 entoderm. This invagination elongates headward in the embryonic area 

 constituting a tube of ectodermal cells, the chordal canal. Later the ventral 

 wall of the canal and the adjacent entoderm disappear, so that the chordal 

 ectoderm temporarily forms the dorsal median boundary of the entodermal 

 cavity. By this process a communication is established between the ento- 

 dermal cavity and neural groove, called the neuro-enteric canal. The chordal 

 ectoderm separates from the entoderm and then forms a solid cord of cells, 

 the notochord; between entoderm and neural groove the neurenteric canal, 

 however, persisting for some time. In the meanwhile, other ectodermic 

 cells in the region of the chordal invagination spread between ectoderm and 

 entoderm and form the anlage of the mesoderm. These cells by rapid prolif- 

 eration soon separate ectoderm and entoderm and join the extra-embryonic 

 mesoderm. The separation of these two structures is complete except in 

 the regions of the bucco-pharyngeal and cloacal membranes. 



On each side of the neural groove the mesoderm becomes transversely 

 grooved in its ectodermal surface forming a number of successive block-like 

 masses called primitive somites or segments; of these, there are thirty-eight 

 for the trunk and possibly four for the head regions. Each segment consists 

 of three parts, the sclerotome, the myotome and the dermatome. Lateral 

 to the somite is a thickened mass of mesoderm, the intermediate-cell mass, 

 that laterally divides into two layers; the outer accompanies the ectoderm 

 forming the somatopleure, which gives rise to the body wall; the inner joins 

 the entoderm, forming the splanchnopleure from which the gut tract, vitelline 

 duct and yolksack are derived. 



Fetal Membranes. As the primitive streak and neural groove are forming, 

 the extra-embryonic mesoderm that lies beneath the trophoderm, invades the 

 trophodermic villi, forming there the chorion with its villi. Gradually the 

 mesoderm of the roof of the amniotic cavity divides into two layers, the upper 

 constituting chorionic mesoderm, while the under one is attached to the ecto- 

 derm of the amniotic, and forms with the latter, the Amnion. In the chick 



