THE EARLY EMBRYOLOGY OF THE MOUSE 



considerable variation, however, and in one study passage into the uterus at 

 4 days was found to be the rule (7). 



The blastula. — Shortly after entering the uterus, and usually sometime 

 after the egg has reached the ,:;2-cell stage, an eccentrically located, fluid 

 tilled cavity appears among the cells of the morula. This enlarges rapidly 







<^ 



t^ ^ 







^^ 



'% 



^ 



uterine lumen 



^^(2) Q^ '*^^ G^- - --- ■ 



Inner cell mass 



Blastocoele 



Uterine epithelium 



Decidua 



B. Behen 



Fig. 3. — Blaatula in uterine crypt 4 days after copulation. Projection drawing 



(X600). 



to produce the segmentation cavity or blastocoele (Fig. 3). The cavity is 

 bounded by only a single layer of cells except on one side where most of the 

 cells are grouped to form a structure called the inner cell mass. Eggs in 

 this stage are known as blastulae. 



Implantation and early growth. — The uterus in the mouse is duplex, con- 

 sisting of two horns which unite just anterior to their junction with the 

 vagina, and each of which is attached to the dorsal body wall by a mesentery, 

 the mesometrium (Fig. 4). There are two layers of muscle in each horn, an 

 outer longitudinal layer and an inner circular layer. The uterine lumen is 

 lined with epithelium. Between the epithelium and the muscle layers is the 

 mucosa, a tissue which forms the bulk of the uterine wall. The epithelium 

 is indented by numerous small crypts. 



