50 



BIOLOGY OF THE LABORATORY MOUSE 



it, the sides of the U are Ukewise brought into approximation and fused 

 together in the mid-ventral Une. The endocardium is thus transformed 

 from a U-shaped structure into a single tube. At the three somite stage (in 

 the rat) the different regions of the heart are not clearly set apart, though a 

 slight constriction serves to mark the boundary between the atrium and the 

 ventricle. As a result of subsequent foldings of the endocardial tube the 

 different regions of the heart are clearly differentiated (Fig. 31). 



Internal carotid artery Aortic arch I Allantois Omphalomesenteric artery 



Atrium 

 Omphalomesenteric vein 



Vitelline 

 arteries 



Yolk-sac 



Fore-gut 



Dorsal aorta Hind-gut 



Fig. 31. — Diagram of the circulatory system in an 8 day 10 hour, 10 somite 

 embryo. The head and tail folds of this embryo have begun to turn but there is as 

 yet no turning in the mid-trunk region. Traces of the allantoic veins are present 

 but are not shown as they do not yet form a continuous channel. (X64.) 



Blood vessels. — In ten somite embryos, still in the process of turning, a 

 number of blood vessels have become established (Fig. 31). The dorsal 

 aorta at this stage is a paired vessel running the length of the trunk. It 

 connects anteriorly with the heart by way of the aortic arches and the ventral 

 aorta. Posteriorly its two halves fuse at the caudal extremity of the hind- 

 gut to form the single, median, omphalomesenteric artery. This runs 

 cephalad for a short distance ventral to the hind-gut and then turns away 

 from the embryo towards the inner surface of the yolk-sac on which it 

 spreads out into a network of capillaries. These capillaries are derived from 



