TEX': LYOLOGY. 



'hich at this time- be< '.>me functional; and is returned to the left atrium by - 

 ulmonary veins. The dnctus artcriosus atrophies to form the ligament 

 rteriosura. From the left atrium the pure blood Hows into the left 

 hence is forced out through the aorta and its brandies to all parts of the bo 

 t the same time the more distal portions of the umbilical arteries in the e. 

 ryo atrophy to form the lateral umbilical ligaments, their proximal porti- 

 ersisting as the superior vesical arteries 245,1 ._ 



Histogenesis of the Blood Cells. 



There is probably no other subject in embryology about which there 

 ore conflicting views than the problem of "the origin and histogenesis of 

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

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



r 



Entoderm 

 Endothelium 



Blood vessel with 

 erythroblasts 



Mesoderm 



FlG^^f/f^T^n section of wail of yolk sac of a human embryo of 5 mm., showing blood 

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



later foetal and postnatal life of the individual. While in some r< 



light has been thrown on the subject, by studies on pathological bl>od co 



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



banges in the blood and disturbances in the blood-formim: 

 problem has in other respects been, complicated by these same stu- 

 ously the questions of the embryonic origin of the blood cells, of their 1101 

 origin during postnatal life, and their origin in abnormal or patholog.- 

 ditions are closely associated. 



Earlier in thi c chapter it was stated (p. 235) that in the periphe: 

 the area opaca certain masses of cells the blood islands appeared in 



