28 BIOLOGY OF THE LABORATORY MOUSE 



invagination has reversed the inversion of the' getfrn layers found in the early 

 egg cylinder. This is the first of the steps by which the germ layers in mice 

 are brought into the relation characteristic of the adult, i.e., entoderm on the 

 inside, ectoderm on the outside, mesoderm in between. 



The hind-gut, less precocious than the fore-gut, appears at about 7^^ 

 days as an invagination in the entoderm and overlying layers at the posterior 

 end of the primitive streak (Fig. 16). 



The open ends of the fore- and hind-guts are eventually joined by the 

 mid-gut whose formation will be described in a later section. It is not these 

 open ends, but the blind ends of the two guts which, by breaking through to 

 the outside, give rise to mouth and anus. An early stage in the development 

 of the mouth may be seen in 8 day embryos (Fig. 22). In the ectoderm of 

 the head there is an invagination directed towards the anterior extremity of 

 the fore-gut. This is the stomodaeum. The wall between the stomodaeum 

 and the fore-gut is the oral plate. In course of time this ruptures and the 

 mouth opening is thereby estabhshed. The anus develops in a similar 

 manner at a somewhat later stage. 



The head fold. — The invagination of the fore-gut involves a pushing or 

 folding of the adjacent tissues into the amniotic cavity. The structure thus 

 produced is the head fold (Fig. 15). First appearing at about 73^ days, it 

 becomes a large and conspicuous structure within less than twenty-four 

 hours (Figs. 22 and 26A). The growth of the neural folds in this region is 

 more rapid than elsewhere, presaging the formation of the brain, and the 

 heart, just ventral to the head fold, is also conspicuous by its rapid growth. 

 In 8 or S}^ day embryos the difference in size between the head region and 

 the middle of the trunk is striking. The head fold is thus the center of a 

 region of particularly rapid growth (24). 



The somites. — Since the somites are mesodermal structures, it will be 

 useful before discussing their development to review the distribution of the 

 mesoderm at the 7^2 dav, stage when they make their first appearance. In 

 the extra-embryonic regidri,the entire exocoelom is lined with mesoderm. 

 The exocoelom contains also the allantois, a wholly mesodermal structure.* 

 In the embryo proper there is little mesoderm in the mid-sagittal region. 

 One small mass which will later contribute to the formation of the heart 



* Not in the exocoelom, but also not part of the embryonic mesoderm, is a small 

 mass of mesoderm at the anterior extremity of the amnion (Fig. 1 5) . This is character- 

 istically in the form of two hollow, thin-walled vesicles, one on each side of the mid- 

 hne, though the range of variation is considerable. That the vesicles are paired is 

 probably due to the fact that the mesoderm in this region grew in from the two sides. 

 Between the two vesicles, and hence approximately in the mid-Une (it may be a little 

 to one side or the other), is a very small area where ectoderm and entoderm are unsepa- 



