42 



BIOLOGY OF THE LABORATORY MOUSE 



precocious growth of certain parts of the neural groove ectoderm indicates 

 early differentiation of the brain. The hind-gut is small, but there is a deep 

 fore-gut, and the heart, just anterior to the fore-gut, is a conspicuous struc- 

 ture. No mid-gut has formed. The allantois has almost reached the 

 chorion, in fact in some embryos at this stage has already reached and fused 



Blood islands 



Exocoelom 



Allantois 



Pericardial 

 coelom 



Epi-myocardium 



Fore-gut 



Neural fold 



Omphalomesen- 

 teric artery 



Hind -gut 



Amniotic cavity 



Splanchnopleure 



Somatopleure 



Undivided 

 mesoderm 



7th Somite 



Fig. 22. — Drawing of reconstruction of 8 day i hour, 7 somite embryo. The 

 reconstruction is cut in the mid-sagittal plane and only the right half shown except 

 at the ventral extremity where the last 4 somites and part of the undivided mesoderm 

 of the left side are included. Cut areas are shown by horizontal shading. ( X 75.) 



with it. The amnion, and the yolk-sac plus the chorion, form a double, 

 arched roof over the whole dorsal surface of the embryo. The blood islands 

 appear as a conspicuous hummocky band around the inner surface of the 

 yolk-sac. Within the embryo blood vessels have begun to form. 



The tail fold. — The hind-gut, though much later to appear than the 

 foregut, soon overtakes it in development. In ten somite embryos the two 

 are of approximately equal size (Fig. 31). A necessary concomitant of 

 hind-gut growth is the appearance of a tail fold; the gut entoderm pushes the 

 overlying ectoderm and mesoderm ahead of it away from the yolk-sac wall. 



