THE EARLY EMBRYOLOGY OF THE MOUSE 



35 



the trophectoderm. Between the two cell layers there soon begins to appear 

 a thin, non-cellular, pink-staining membrane called, after the man who first 



'Ectoplacenta 



Reichert's 

 membrane 



Yolk-sac 



Amnion 



Exocoelom 



Mid -gut 



Blood island 



Ectoplacenta 



Neural tube 



Allantois 



Exocoelom 



Yolk-sac 



Reichert's 

 membrane 



Mid-gut Dorsal aorta 



Fig. 20. — Frontal sections showing development of extra-embryonic membranes 

 and formation of mid-gut. The sections are viewed from the head towards the tail, 

 so that the right side of the embryo is on the left side of the drawing, and vice versa. 

 .\. 8 days 11 hours, ii somites, through oth somite. Reichert's membrane omitted. 

 (X60.) B. S days 18 hours, 16 somites, through oth somite. (X45.) 



described it, Reichert's membrane. The first signs of it are often visible at 

 the ventral extremity of the egg where there is apt to be a cluster of entoderm 



