THE HISTOGENESIS OF DENSE LYMPHATIC TISSUE 191 



the formation of erythrocytes. These erythropoietic foci are 

 found mainly in siibnodular and internodular connective tissue 

 of the submucosa, just the opposite of the principal granulo- 

 poietic foci which are, in large part, found in the tunica propria 

 above and between the lymphatic nodules. Erythropoietic foci 

 are also of more frequent occurrence, and more extensive when 

 present, in the region of Peyer's patch than in the appendix, 

 again just the opposite of the granulopoietic activity. 



Scattered erythroblast cells were also found in the subnodular 

 and internodular connective tissue in stages both younger than 

 two weeks and older than six weeks, but only during this period 

 were erythropoietic foci of any extent found. 



This close assciation between erythropoiesis and lymphoiesis 

 is of exceptional interest because of the exceeding rarity of red- 

 cell formation in connection with lymphatic tissue. One case 

 of erythropoiesis occurring in lymphatic nodes was described by 

 Pappenheim. This was found under pathological conditions, 

 however, being associated with an acute case of hemorrhagic 

 macrolymphocytic leukemia. Scattered erythroblasts have also 

 been noted from time to time in hemolymph nodes. In the ileum 

 and occasionally in the appendix of the rabbit one finds eryth- 

 ropoiesis occurring in no uncertain manner under apparently nor- 

 mal conditions in close relationship with the lymphatic tissue 

 there present (figs. 7, 14, 15). 



The developmental history of the three types of blood cells 

 and possible relations between them has long been a perplexing 

 question. The "relation in origin between cells of the erythrocyte 

 and granulocyte series has been particularly a matter of spirited 

 controversy. On this question hematologists are divided in 

 opinion into two groups. 



The first group, believing in a monophyletic origin of blood 

 cells, consider the erythrocytes and granulocytes are derived from 

 a common stem cell, which, under certain environment, will 

 develop in to the other type of blood cell. 



The second group, on the other hand, believing in a polyphy- 

 letic origin of blood cells, consider that all types of cell of the 

 blood series arise from stem cells of different character, which, 



