20 BIOLOGY OF THE LABORATORY MOUSE 



is the amniotic cavity, lined with embryonic ectoderm.* At this stage it is 

 cup-shaped, as can be seen from Figs. 13 and 14A, which show it in sagittal 

 and cross section respectively. 



In the middle is the exocoelom, lined with mesoderm. 



At the dorsal extremity of the egg cylinder is the ectoplacental cavity, 

 lined with extra-embryonic ectoderm. The amniotic cavity and the exo- 

 coelom, though neither one, as will be seen later, is included in the actual 

 body of the embryo, are important in its future development. The ecto- 

 placental cavity, on the other hand, already the smallest of the three, 

 gradually becomes narrower and finally disappears. 



The membrane separating the amniotic cavity from the exocoelom is 

 called the amnion. It is composed of two thin, cellular layers, one of 

 ectoderm, the other of mesoderm. Separating the exocoelom from the 

 ectoplacental cavity is another membrane, the chorion, likewise composed of 

 ectoderm and mesoderm. 



The head process, f — It will be remembered that mesoderm is first 

 proliferated by the primitive streak in embryos about 6^^ days old. The 

 growth is entirely from the lateral and caudal margins of the primitive 

 streak; no mesoderm is proliferated from its cephalad extremity. Beginning 

 at about 7 days, however, growth does occur in this region, but the structure 

 formed shows greater affinity to the entoderm than to the mesoderm. It is 

 known as the head process. In sagittal sections it first appears as a wedge 

 shaped group of cells between the ectoderm and entoderm at the ventral 

 extremity of the egg cylinder (Fig. 12). The base of the wedge is attached 

 to the ventral end of the primitive streak from which it takes its origin; the 

 tip of the wedge points forward towards the anterior margin of the egg 

 cylinder. Cells grow out rapidly from the margins of the wedge, forming a 

 thin, spreading sheet between ectoderm and entoderm. J 



* We interpret the ectoderm of the amnion as embryonic ectoderm, the ectoderm 

 of the chorion as extra-embryonic ectoderm. The evidence on this point is not neces- 

 sarily conclusive, however, for concurrent with the appearance of the amniotic folds, 

 the division between the two types of ectoderm loses much of its distinctness. 



t Sobotta (62) studied embryos representing the stages during which the head 

 process develops, but his drawings indicate that his otherwise admirable sections were 

 not close enough to the exact sagittal plane to reveal this structure clearly. Conse- 

 quently, it remained for Jolly and Ferester-Tadie (26) to first describe it correctly 

 for the mouse and rat. Our observations are entirely in accord with theirs. 



t The entoderm and the margins of the head process are so thin and close together 

 at this stage that favorable conditions are necessary to distinguish them. In the 



