102 



THE EARLY DEVELOPMENT OF MAMMALS. 



The descending and median aortae give off intersegmental vessels, some of 

 which persist in the adult. The main aorta produces three main branches on its 

 ventral side, the histories of which are somewhat complicated. They are the 

 coeliac axis, the superior mesenteric, and the inferior mesenteric. 



The Venous System. 



The veins do not for the most part, if at all, arise as independent vessels, but 

 by the transformation of channels in a network of small vessels. Arteries develop 

 in the same way, but with them the growth of the main stem as such plays a 



FIG. 57. CHICK EMBRYO WITH SEVENTEEN SEGMENTS. DRAWN FROM A SPECIMEN WHICH HAD BEEN AUTO- 

 INJECTED WITH INDIA-INK. (After H. M. Evans.) 



greater role. The method of development is illustrated by figures 57 and 58, From 

 the first aortic arch a network of small vessels spreads into the head at the sides 

 of the fore-brain and mid-brain, gradually occupying an increasing territory- until the 

 whole head is supplied. The plexus forms a single vascular layer between the brain 

 and epidermis and is at first without any main channels (Fig. 57). Soon some of 

 the capillaries enlarge, forming a branching system (Fig. 58), the branches leading 

 into a main stem which extends from the head to the posterior or venous end of 

 the heart. This stem, which rapidly increases in size, is the anterior cardinal vein, 



