194 JOHN STEPHENS LATTA 



also, in Fundulus, that erythropoiesis occurred only extravas- 

 cularly, and that as soon as the erythropoietic tissue became 

 included in a vessel, the process shifted to other regions of the 

 embryo. The intravascular conditions he considered inadequate 

 and inhibitory, rather than active factors for erythropoiesis. 



As a partial solution of this apparent discrepancy in mammals 

 might be advanced the results of Mollier ('13) in his study of 

 erythropoiesis in the human embryonic liver. He found the 

 endothelial walls of the sinusoidal vessels to be reticulated so 

 that communication was made between the lumina of blood 

 vessels and the immediately surrounding mesenchymal spaces 

 in which the erythrocytes develop. Thus conditions in these 

 spaces would be essentially intra- rather than extravascular. 



Secondary extravascular erythropoiesis was also found to 

 occur in birds (chick) by Danchakoff in the allantois following 

 splenic grafts upon it. Some of the walls of allantoic blood 

 vessels degenerated allowing the contents of the vessel, early 

 stages of erythroblasts, to wander out into the mesenchyme 

 forming extravascular erythropoietic foci. The cells thus liber- 

 ated did not revert to lymphoid hemoblasts, but continued their 

 development as begun intra vascularly. 



Using Maximow's experiments on the formation of bone- 

 marrow in the kidney of the rabbit following ligature of the renal 

 vessels, she applies this theory to mammals. Maximow found 

 large groups of lymphoid hemoblasts collecting in the vessels due 

 to the slower current caused by ligation which began erythropoi- 

 etic differentiation intra vascularly, and wandered out into the 

 surrounding tissue at the normoblast stage. With this evi- 

 dence at hand she thinks it probable that all extravascular eryth- 

 ropoiesis, whenever found, is only secondarily so, the process 

 outside of the vessels being a homoplastic difTerentiation of 

 specific cells (erythroblasts), irreversible in their development. 



Erythropoiesis, as occurring in the submucosa in the region of 

 Peyer's patch, in the rabbit, is definitely extravascular Eryth- 

 ropoietic foci of very large proportions are often found at 

 certain developmental stages of the tonsillar tissue, sometimes 

 apparently filling most of the available space in the subnodular 



