EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 277 



Numerous basophilic cells differentiate at the expense of the 

 syncj^tium and become ameboid (figs. 5 and 6, L. Hhl.). Acido- 

 phylic granules appear in the cytox:>lasm of a part of these cells 

 and characterize them as granulocytoblasts (fig. 8, Grbl.). The 

 development of these cells (lymphoid hemocytoblasts and granu- 

 locytoblasts) is not specific for the spleen mesenchyme and is 

 observed in other regions of the embryo body also. At the same 

 time the spleen anlage becomes vascularized, and its vasculariza- 

 tion is at this stage of its development exclusively veinous. In 

 the peripheral layers of the anlage, later on through the whole 

 organ, appear splits, which evidently are filled by a liquid, 

 which separates the cells. These splits are at first surrounded by 

 the irregular surface of the mesenchymal cells. Some of the cells 

 show still their processes, projected in the lumen of the sinuses, 

 (fig. 6, S.). These sinuses soon join together and form a net. 

 From the other hand a communication with branches of the 

 intestinal veins is established. The whole mesenchymal anlage 

 exhibits at this second stage of its development a spongious 

 structure. Whether the appearance of the splits in the anlage 

 is due to a secretion of the surroimding cells, or to a transudation 

 of a liquid through the vessels growing from outside is difficult 

 to determine. There is, however, no doubt that the splits 

 mentioned are of local origin. This has been shown by Laguesse 

 (23) in fishes. 



The appearance of the splits in the tissue of the spleen anlage 

 is accompanied by more intense isolation of ameboid cells 

 (fig. 6, L.Hhl"). Some of them are surrounded by a developing 

 sinus and become situated in its lumen. These cells have 

 invariably first the structure of lymphoid hemocytoblasts (large 

 lymphocytes). Similar cell groups are not seldom encountered 

 in the larger sinuses of the peripheral layers in the spleen, (fig. 5, 

 *S). As soon, however, as these lacunae unite with the veinous 

 vessels, what is indicated by the sudden appearance of differ- 

 entiated erythrocytes within the lacunae, the lymphoid hemo- 

 cytoblasts begin here their differentiation into erythroblasts (fig. 

 5, Erhl.). The plasma of the blood must evidently contain 

 factors for differentiation of lymphoid hemocytoblasts into ery- 



