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TEXT-BOOK OF EMBRYOLOGY. 



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 em- 

 bryo atrophy to form the lateral umbilical ligaments, their proximal portions 

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



Histogenesis of the Blood Cells. 



There is probably no other subject in embryology about which there are 

 more conflicting views than the problem of the origin and histogenesis of the 

 blood cells. The problem concerns not only the first blood cells in the embryo 

 and their life history, but also the origin and development of new cells during 



Entoderm 

 Endothelium 



Blood vessel with 

 normoblasts 



Mesoderm 



FlG. 246. From section of wall of yolk sac of a human embryo of 5 mm., showing blood 

 vessel containing nucleated red blood cells (normoblasts). Photograph. 



later fcetal and postnatal life of the individual. While in some respects much 

 light has been thrown on the subject, by studies on pathological blood condi- 

 tions such as anaemia, leucocytosis, leukaemia, and other conditions accompanied 

 by changes in the blood and disturbances in the blood-forming organs, the 

 problem has in other respects been complicated by these same studies. Obvi- 

 ously the questions of the embryonic origin of the blood cells, of their normal 

 origin during postnatal life, and their origin in abnormal or pathological con- 

 ditions are closely associated. 



The latest theory of the development of the blood cells is the result of an 

 extensive series of investigations by Maximow, with whom Weidenreich, 



