26 



BIOLOGY OF THE LABORATORY MOUSE 



tinuous structure is established connecting the posterior end of the primitive 

 streak with the ectoplacental cone. In due course embryonic blood vessels 



ectoplacental cone 



ectoplacental cavity 

 chorion 



yolk sac 



entoderm 



archenteron 



Fig. 15. — Sagittal section of mouse embryo of 7 days 15 hours. Reichert's membrane 

 omitted. Projection drawing (XiSo). 



will find their way along this pathway to make contact with the maternal 

 blood supply. 



Fore -gut and hind -gut. — In the early stages of its formation the digestive 

 tract consists of three quite distinct parts, the fore-gut, the hind-gut and the 

 mid-gut. These appear in the order named. The fore-gut can be traced 



