104 JOHN STEPHENS LATTA 



with its formation in the subnodular and internodular connec- 

 tive tissue, groups of cells in developmental stage of the forma- 

 tion of erythrocytes (erythroplastids) . These erythroblastic cell 

 groups are not of constant appearance, and, when present, vary 

 greatly in number, sometimes there being but a few erythroblas- 

 tic cells scattered about in the connective tissue and, again, 

 large masses of them, or foci, were found. These erythropoietic 

 foci were found in largest extent in the region of the iliac tonsil 

 (Peyer's patch), but in a few cases erythropoiesis was noted to be 

 occurring to a very limited extent in the connective tissue of the 

 wall of the appendix (figs. 7, 14, 15). 



This phenomenon of erythropoiesis is not confined to any exact, 

 definite period of development, but it does, however, occur most 

 frequently and in greatest amount in animals from two to six 

 weeks after birth. Even though, as before stated, the presence 

 of erythropoietic centers is not of absolute constancy, they are 

 found present so frequently (and in such number) as to render 

 improbable the thought that they arise because of some partic- 

 ular pathological condition in each case. 



It is apparent, therefore, that in the study of the histogenesis 

 of elements of the connective tissue in the region of the intesti- 

 nal tonsils of the rabbit, one is confronted with the problems con- 

 cerned with the formation of all the types of blood cells, which may 

 be divided into three phases, according to the character of the 

 resulting cells: 1) lymphopoiesis, the development of the com- 

 ponents of the lymphatic tissue, the lymphocytes and reticulum; 

 2) the development of cells containing in their cytoplasm 

 acidophilic granules, or granulopoiesis, and, 3) the development 

 of red blood cells, or erythropoiesis. The origin, development, 

 and the fate of these three different types of cells, and conditions 

 causing and controlling their production, as well as interrela- 

 tions which might exist between the three phases of develop- 

 ment, are questions for which this material gives opportunity 

 for study. 



