THE PKIMITIVE VASCULAE SYSTEM 



65 



1st aortic 

 arch 



Common trunk formed 



by umbilical and 



yolk-sac veins 



ena umbilicalis 

 impar 

 Umbilical arteries 



Vitelline Arteries 



The primitive mesanioeboids are formed in the wall of the yolk-sac, and 

 there some of them produce erythrocytes ; many, however, migrate into the 

 embryo, where some of them take part in the formation of the walls of the em- 

 bryonic blood-vessels, and others become enclosed in the liver, the lymph glands, 

 and the bone marrow, where they become foci for the formation of blood corpuscles. 



During the first two months the primitive forms of red blood cells predominate. 



In the Second month Dorsal intersegmental branches 



the sauroid cells in- Dorsal aort* 



crease considerably 

 in number, and from 

 the third month the 

 blood plastids become 

 more and more 

 numerous, until, at 

 the eighth month 

 (Minot), the majority 

 of the blood cells 

 are blood plastids 

 undergoing conver- 

 sion into blood cor- 

 puscles. At this time 

 the colourless cells 

 are present in a very FIG. SI. SCHEMA OF CIRCULATION OP AN EMBRYO, 1*35 MM. LONG, WITH Six 



distinct minority. SOMITES - < After Felix ' modifie(L) 



Formation of the Primitive Blood Vascular System of the Embryo. The earliest 

 stage of the formation of the heart and blood-vessels in the human subject are not 

 known, but, judging by what occurs in other mammals, it is probable that the first- 

 formed vessels appear in the splanchnic mesoderm before the embryonic area 

 begins to fold. It is presumed that they are formed by aiigioblastic cells which 

 have migrated into the embryonic area from the walls of the yolk-sac. From 

 their seat of origin they extend towards the caudal end of the embryonic area, one 

 on each side of the notochord, and from the caudal end of the embryonic region 

 they pass along the body-stalk into the chorion. (See note 5, p. 79.) 



As the cephalic 



Dorsfal intersegmental branches end of the embryonic 



aort8e area is folded, to 



enclose the fore-gut, 

 the corresponding 

 parts of the primi- 

 tive arteries are bent 

 into a c-shaped form. 

 The ventral limb of 

 the c, which lies in 

 the dorsal wall of the 

 pericardium and the 

 ventral wall of the 

 fore-gut, is the primi- 

 tive ventral aorta. The 

 bend of the c is the 

 first aortic arch, which 

 passes along the 

 lateral margin of the 



bucco-pharyngeal membrane. The dorsal limb of the c is the cranial part of the 

 primitive dorsal aorta. The primitive dorsal aorta passes posteriorly into the tail 

 and gives off in the region of the tail fold the primitive umbilical artery, which runs 

 along the body-stalk to the chorion. 



The caudal parts of the primitive ventral aortae are the rudiments of the heart. 

 At first they lie, quite separate from each other, in the dorsal wall of the pericardium, 

 but soon they approach one another and fuse together to form a single tubular 



5 . 



Anterior cardinal 

 ve; 



1st aortic arch 



Heart 



Stem formed by union of 



lateral umbilical and 



vitelline veins 



| Vena umbilicalis 



impar 

 Umbilical arteries 



Vitelline veins 



FIG. 82. SCHEMA OF VASCULAR SYSTEM OF AN EMBRYO, 2 '6 MM. LONG, WITH 

 FOURTEEN SOMITES. (Arteries after Felix, modified.) 



