THE HISTOGENESIS OF DENSE LYMPHATIC TISSUE 195 



and internodular connective itssue. Most of these foci do, it is 

 true, contain cells in the later stage of erythroblastic development 

 (normoblasts). But megaloblastic, extravascular foci may also 

 be found. Cells in the same group are often at various stages of 

 erythroblastic development (figs. 8, 14, 15). 



If this extravascular erythropoiesis were secondarily so, one 

 would expect to find at some developmental stage intravascular 

 megaloblastic foci, or at least intravascular groups of lymphoid 

 hemoblasts, but at no time were such found. The entire process 

 of erythropoiesis as it occurs here is apparently extravascular. 



No evidences were found to indicate that the walls of the 

 submucosal blood vessels were reticulated so as to make the mes- 

 enchymal spaces connected with the lumina of the blood vessels. 

 This is true extravascular erythropoiesis, and not secondarily so, 

 nor occurring under conditions of intravascularity (as Mollier 

 described). 



Though this erythropoiesis is extravascular, erythropoietic 

 foci are always seen to be in very close relationship with the blood 

 vessels of the submucosa. There is little doubt but that some 

 action of the plasma or some element contained in it upon lym- 

 phoid hemoblasts locally developed incites them to further 

 differentiation along the erythroblastic line of development. 

 This seems to be very similar to the granulopoietic relations 

 before described, but it is found that this relationship between 

 erythropoietic foci and the blood vessels is much closer than be- 

 tween the granulopoietic foci and the blood vessels. There 

 certainly is, however, very great similarity between granulo- 

 poiesis and erythropoiesis, and only a slight difference in envi- 

 ronmental conditions must exist. 



Both granulopoiesis and erythropoiesis are found to occur in 

 largest extent when the blood vessels are gorged with erythro- 

 cytes (i.e., when the current in the blood stream is slow). If one 

 considers the various places in the body of the embryo and adult 

 where these two phenomena are known to occur, as in the liver, 

 spleen, bone-marrow, etc., these are found to be invariable places 

 where the blood current is slow, the slowing of the current usually 

 being due to the sinusoidal nature of the vessels. This slowing 



