THE EARLY EMBRYOLOGY OF THE MOUSE 



7 



back to the 7 day stage when it consists merely of a deep notch in the thick 

 or columnar entoderm at the anterior margin of the egg cylinder (Fig. 12). 

 Six hours later there is Httle change (Fig. 13), but by 'jYi days (Fig. 15) the 

 notch has been replaced by a definite pocket in the entoderm, and the 

 entoderm surrounding the pocket together with the overlying ectoderm form 

 a bulge which projects into the amniotic cavity. From this stage on, 



^^^ -Ectoplacenta 



Reichert's 

 membrane 



Ectoplacental 

 cavity 



Exocoelom 

 Amnion 



Somatic mesoderm 



Pericardial coelom 

 Epi-myocardium 



Endocardium 

 Fore-gut 



Amniotic cavity 



Chorion 



.\llantois 



Yolk-sac 



Blood island 



Omphalomesen- 

 teric artery 



Hind-gut 



Primitive streak 



Ectoderm 



&*'" ^Notochord 



Fig. 16.— Partly diagrammatic sagittal section of embryo of 7 days 18 hours (Xioo). 



Reichert's membrane omitted. 



growth of the fore-gut is exceedingly rapid, the pocket changing in a few 

 hours into a deep pouch (Fig. 16). The process is due to a progressive 

 drawing together in the mid- ventral line of the folds of entoderm that bound 

 the anterior intestinal portal (Figs. 25C and 30), the portal thus being shifted 

 farther and farther towards the cudal extremity of the embryo. The process 

 has been aptly described as a "zipper action." 



It should be noted that the fore-gut is lined by entoderm, and that this is 

 surrounded by mesoderm and ectoderm. Thus in this region the process of 



