60 



Umb. vesicle of 13 mm h. embryo 10 to 15 „ 



It seems more probable that the cells of the liver, actively pro- 

 liferating in the capillary sinuses of this favorable organ for hemato- 

 poiesis, are swept there from the yolk sac. This is the more likely 

 since nothing corresponding to the extravascular cell-balls described 

 by ScHRiDDE can be demonstrated. According to Schmidt (41) the 

 endothelial cells of the liver capillaries produce, by a process of in- 

 ternal and external proliferation, the primitive lymphocytes, which 

 subsequently differentiate into red blood cells. Occasionally an endo- 

 thelial cell can be seen rounding up into a blood cell in the liver 

 of the 13 mm embryo, but it is an intravascular process. Rarely 

 also a cell resembling the lymphocyte of the yolk sac can be seen 

 among the liver cells. They appear to arise from the undifferentiated 

 mesenchyme between the liver columns. Schridde denies the presence 

 of such mesenchyme, but, in view of the manner of the development 

 of the liver it seems more reasonable that mesenchyme should persist 

 in latent and undifferentiated condition for the continued manufacture 

 of normoblasts at the time when the liver becomes the essential 

 hematopoietic organ of the embryo i). In the 13 mm embryo the liver 

 seems just entering upon its hematogenous function. 



1) S. Mollier's recent very important study of hepatic hemato- 

 poiesis ("Die Blutbildung in der embryonalen Leber des Menschen und 

 der Säugetiere", Archiv f. mikrosk. Anat. u. Entwickig., Bd. 74, Heft 3, 

 p. 474 — 524), in the main confirms the findings of Maximow and contra- 

 dicts those of Schridde. The liver is described as predominantly an 

 erythropoietic organ ; nevertheless, from the beginning it is also to some 

 degree a leucopoietic organ giving origin to mononuclear leucocytes, 

 especially eosinophiles, which secondarily become polymorphonuclears. 

 The liver is probably not also a lymphopoietic organ, though Ij^mpho- 

 cytes, in common with erythrocytes and leucocytes, are said to trace 

 their ancestry back to a common mother cell, the "hemagonium". Mollier 

 thus holds to the monophyletic position. Nevertheless, he demands further 

 histogenetic and experimental research to determine how far the early 

 proliferation products of the hemagonia are identical. Mollier seems 

 more inclined to regard the erythropoietic, leucopoietic and lympho- 

 poietic series as distinct and separate lines subsequent to the hem- 



