696 TEXT-BOOK OF PHYSIOLOGY. 



into which the ovum sinks. As the ovum increases in size the mucosa 

 gradually covers it; that portion of the mucosa toward the uterine cavity is 

 called the decidua capsularis (d. reftexa) , that beneath the ovum the decidua 

 basilaris (placental d.), while the remainder constitutes the decidua parietalis 

 (d. vera). As development proceeds the decidua basilaris becomes greater, 

 ultimately developing into the placenta. 



Segmentation of the Ovum. Immediately after fertilization the ovum 

 divides and redivides, within the diminishing zona pellucida, forming an 

 irregular mass called the morula. The peripheral cells form a layer, the 

 trophoderm, beneath the attenuated zona pellucida ultimately replacing 

 that structure. The remaining cells of the morula differentiate into three 

 masses ectodermal, entodermal and mesodermal; the central cells of these 

 masses liquefy and disappear forming thus the ectodermal or amniotic 

 cavity, limited by the ectoderm; the entodermal cavity, limited by the ento- 

 derm; and the mesodermal or celomic cavity, limited by the extra-embryonic 

 mesoderm. Meanwhile cells in various parts of the thickened trophoderm 

 have disappeared leaving this layer in the form of delicate trophodermal 

 villi, the future chorionic and placental villi. 



The Embryonic Shield. The floor of the amniotic cavity, consisting 

 of ectoderm and entoderm, constitutes the embryonic shield or disk. As 

 the shield increases in size a median longitudinal thickening is seen 

 occupying the caudal half of the area. This is the primitive streak, a tem- 

 porary structure that is soon overshadowed by changes in the areas just in 

 front of it. Here is formed a median, longitudinal, grooved ridge of ecto- 

 derm, that develops rapidly in length. This is the neural groove and folds. 

 The dorsal lips of the groove approach each other in the mid-line and fuse, 

 separating from the original ectoderm which closes over the ectodermal 

 tube. This ectodermal tube is the neural tube from which the nerve system 

 is developed. 



In the immediate vicinity of the head end of the primitive streak is seen 

 a darkened area, Hensen's node that represents the beginning invagination 

 of the ectoderm in the formation of the embryonic mesoderm and notochord 

 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- 



