278 VEEA DANCHAKOFF 



throcytes. The slowness of the blood current in the large 

 veinous capillaries and sinuses offers moreover favorable condi- 

 tions for this line of differentiation. 



The vascularization of a normal spleen proceeds, however, 

 gradually. The cells, surrounding the splits, become gradually 

 flattened and finally form an even endothelial surface (fig. 8). 

 The sinuses of a normal spleen contain usually merely a few 

 young cells undergoing an erythroblastic differentiation. I do not 

 think it right therefore to consider the normal embryonic chick 

 spleen as an active erythropoietic organ, though potentially it 

 must be considered as such. It may be noticed that spleens of 

 embryos at the same stage may offer in this respect well pro- 

 nounced individual differences. New sinuses continue to appear 

 with the growth of the spleen. In later stages, at the Uth 

 day of incubation, as the fig. 7 shows, in newly formed 

 sinuses there may be . found large groups of lymphoid 

 hemocytoblasts, which soon undergo an erythroblastic differ- 

 entiation. Between the new formed vessels the mesenchyme 

 continues to proliferate and to split off lymphoid hemocytoblasts. 

 They multiply also and partly differentiate into granulocytoblasts, 

 which increase also their number by mitosis and partly differen- 

 tiate further into granulocytes (figs. 8 and 9) . 



The processes of growth and differentiation proceed slowly in a 

 normal spleen. In an embryo of 9 days — 5 days after the begin- 

 ning of its development — the size of the spleen reaches 1— 1.5mm 

 the long diameter. The abundance of nuclei however may serve 

 as index of intense proliferation processes taking place in the 

 syncytial tissue of the embryonic spleen. The spleen during its 

 II stage of development is characterized by a development of a 

 net of wide veinous capillaries developed in the mesenchymal 

 syncytium,, by an intense granulopoiesis outside the vessels and 

 by a potential erythropoiesis within the vessels. These lines 

 of differentiation, as known, are also characteristic of the mye- 

 loid metaplasis in the spleen pulpa. The study of the normal 

 spleen development leads to the conclusion that the first differ- 

 entiation processes of the mesenchymal spleen anlage transform 

 it into a pulpa-like organ (figs. 7 and 8). The spleen remains 

 pulpa-like until the 12 to 13 day. 



