DEVELOPMENT OF THE CIRCULATORY SYSTEM. 247 



About the end of the 3rd week the dorsal aortae fuse together 

 from about the region of the 4th dorsal segment to the 4th 

 lumbar. In front of and behind these points they remain 

 separate. 



The fate of the two dorsal aortae in front of the 4th dorsal 

 vertebra has been already dealt with (p. 35 and Figs. 28 and 

 29). It remains to consider what becomes of them in the pos- 

 terior extremity of the body. Following the researches of Young 

 and Eobinson, the dorsal aortae beyond the 4th lumbar segment, 

 where they remain apart, form : 



(1) The common iliac arteries; 



(2) The internal iliac — in part at least ; 



(3) The hypogastric arteries — intra-abdominal parts of um- 

 bilical ; 



(4) The umbilical arteries — in the umbilical cord. 



The authors just mentioned have described in the embryos 

 of several mammals a system of arterial arches on each side 

 of the hind gut resembling the aortic arches on each side of 

 the fore gut (see Fig. 221, p. 273). 



Red Corpuscles of the Blood. — The first red corpuscles are 

 nucleated, capable of division, and are formed in mesoblastic cells 

 of the splanchnopleure of the yolk sac and in the somatopleure 

 of the amnion and chorion. The nucleated red corpuscles 

 gradually become the biconcave, non-nucleated disc, but even 

 at birth nucleated red corpuscles are still present. In the early 

 half of foetal life the liver plays a leading part in their forma- 

 tion ; in the latter half the spleen, red marrow and connective 

 tissues become the chief sites of their formation. After birth 

 they are formed almost entirely in the red marrow. 



White blood corpuscles are produced at a later date than the 

 red blood corpuscles. They also are produced by cells of the 

 mesoblast. 



Lymphatic Vessels.- — Lymphatics are developed in the chick by 

 the end of the 3rd day. It is some time before the lymphatic 

 system opens into the blood system by the formation of a thoracic 

 duct. 



Certain important changes take place in the formation of the 

 human lymphatic system about the 5th month. Lymphatic 

 glands then begin to form in the connective tissue throughout the 



