REPRODUCTION 



749 



to be considered 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 embry- 

 onic 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 ecto- 

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

 streak represents an invagination (gastrulation) of ectoderm between ecto- 

 derm and entoderm. This invagination elongates headward in the embry- 

 onic 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 entodermal cavity and neural groove, called the neurenteric 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 meantime other ecto- 

 dermal cells in the region of the chordal invagination spread between ecto- 

 derm and entoderm and form the anlage of the mesoderm. These cells by 

 rapid proliferation soon separate ectoderm and entoderm and join the 

 extra-embryonic mesoderm. The separation of ectoderm and entoderm is 

 complete exceptiin the regions of the bucco-pharyngeal and cloacal membranes. 



Upon each sde of the neural groove the mesoderm becomes transversely 

 grooved on its ectodermal surface, forming a number of successive block- 

 like masses called primitive somites or segments. Of these there are thirty- 

 eight of the trunk and possibly four 

 for the head region. Each segment 

 consists of three parts the sclero- 

 tome, the myotome and the dermatome. 

 Lateral to the somite is a thickened 

 mass of mesoderm, the intermediate 

 cell-mass, that laterally splits into two 

 layers, the outer accompanies the ecto- 

 derm 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 yolk sac are 

 derived. 



Fetal Membranes. As the primi- 

 tive streak and neural groove are form- 

 ing, the extra-embryonic mesoderm 

 that lies beneath the trophoderm invades the trophodermal villi forming thus 

 the chorion with its villi. Gradually the mesoderm of the roof of the am- 

 niotic cavity splits into two layers, the upper constituting chorionic meso- 

 derm while the under one attached to the ectoderm of the amniotic forms 

 with the latter the amnion. In the chick and some mammals, the amnion 

 is derived from the somatopleure in the folding off of the body. In amniotes 

 the amniotic cavity is at first small but rapidly increases in size. It con- 

 tains a clear, transparent liquid, the amniotic fluid, which amounts to about 



FIG. 329. HUMAN EMBRYO AND ITS EN- 

 VELOPES AT THE END OF THE THIRD MONTH. 



(Dalton.) 



