THE DEVELOPMENT OF THE VASCULAR SYSTEM 267 



receives purer blood than any other part of the body, and this is undoubtedly 

 correlated with the relatively enormous size of that organ in the foetus. 

 The rather impure blood which starts through the dorsal aorta is in part 

 distributed to the viscera, body walls, and lower extremities by the visceral 

 and segmental arteries, and thence is collected by the branches of the portal 

 vein and inferior vena cava to be returned as impure blood to the umbilical 

 current at the liver; in part it is carried by the umbilical arteries to the pla- 

 centa, there to be purified and collected by the branches of the umbilical 

 vein (see Fig. 244). 



At birth, when the placental circulation is cut ofT, the proximal end of the 

 umbilical vein atrophies to form the round ligament of the liver; the ductus 

 venosus also atrophies and becomes merely a connective-tissue cord in the 

 liver. The hepatic portal circulation is still maintained by the portal vein. 

 The foramen ovale is closed and the impure blood from the inferior vena cava 

 as well as that from the superior, passes from the right atrium into the right 

 ventricle and thence is forced out through the pulmonary artery to the lungs, 

 which at this time become functional, and is returned to the left atrium by the 

 pulmonary veins. The ductus arteriosus atrophies to form the ligamentum 

 arteriosum. From the left atrium the pure blood flows into the left ventricle, 

 thence is forced out through the aorta and its branches to all parts of the 

 body. At the same time the more distal portions of the umbilical arteries in 

 the embryo atrophy to form the lateral umbilical ligaments, their proximal 

 portions persisting as the superior vesical arteries (see Fig. 245). 



Haemopoiesis Histogenesis of the Blood Cells. 



Two sharply contrasting views are held regarding the origin and genetic 

 relationships of the different kinds of blood cells. The one view, expressed 

 in the monophyletic theory, holds that there is differentiated out of the mesen- 

 chyme a certain type of cells the primitive blood cells, or haemoblasts 

 and that from this single type all the cells of the blood arise through proc- 

 esses of development along divergent lines. The other view, expressed in 

 the polyphyletic theory, holds that while the blood cells are of mesenchymal 

 origin the red cells and white cells have a dual origin, each type arising from 

 its own mother-cells ; and further that perhaps each kind of white cells arises 

 from a distinct parent-cell. The recent extensive studies of the problem 

 have yielded evidence that turns the balance at present in favor of the mono- 

 phyletic theory, and the following account is based in the main upon these 

 studies, particularly those of Maximow on the rabbit and Dantschakoff on 

 the chick. 



The sites of blood formation, or haemopoiesis, are (i) the area opaca 



