DEVELOPMENT OF THE MAMMALIAN SPLEEN 303 



cells in approximately the same stage of differentiation were 

 located in the larger vessels at the periphery of the organ. If 

 the vessels containing these groups of cells were followed through 

 a series of sections, they were seen to communicate with vessels 

 in the mesentery of the spleen. The most striking feature was 

 the absence of the large parent cells that are seen in the regions 

 of differentiation in the central portion of the organ. By far the 

 greatest number of the cells that compose such intravascular 

 groups are normoblasts or erythroblasts with the typical 'Rad- 

 kern' nucleus. Cell division is very pronounced, many mitotic 

 figures being seen in cells that are strongly acidophilic. Such ob- 

 servations might be interpreted, at first glance, as indicating the 

 presence of erythropoiesis without the participation of the local 

 tissue. However, by tracing the vessels through a series of sec- 

 tions, they were found to connect with the enlarged sinuses within 

 the organ and with the larger veins in the mesentery. It is prob- 

 able, therefore, that the cells had begun their differentiation 

 within the central portion of the organ, where erythropoiesis was 

 very active, and that they had made their way through the mes- 

 enchymal tissue into the sinuses, and from here to the larger ves- 

 sels that leave the organ. 



Although the entire process of local differentiation of mesen- 

 chymal cells into hemocytoblasts and these in turn into erythro- 

 cytes, can be readily observed within the splenic tissue, yet one 

 cannot entirely disregard the possibility of an accompanying dif- 

 ferentiation of elements brought in by the blood-stream. In a 

 6-cm. pig an occasional erythroblast is observed in the peripheral 

 circulation. To deny the possibility of their proliferation and 

 differentiation in the retarded blood currents would be contrary 

 to well-established observations. Their scarcity in number, how- 

 ever, would not make it possible for them to play an appreciable 

 role in the hemogenic process within the organ. 



According to Danchakoff, the spleen of the chick embryo is 

 not an active erythropoietic organ. The sinuses of a normal 

 spleen contain a few young cells undergoing erythroblastic dif- 

 ferentiation, but the process never becomes very marked. In 

 mammals, on the other hand, numerous authors have reported 



