306 GEO. A. THIEL AND HAL DOWNEY 



also carried into the blood-vessels in this manner and many of 

 them migrate through the endothelial membrane. In the early 

 stages, while the marrow is still of the primary, lymphoid type, 

 these lymphocytes differentiate, into erythrocytes within the ves- 

 sels. A similar mode of entrance of erythrocytes from the blood- 

 forming tissue to the vessels was described by Lobenhoffer and 

 H. Fischer. 



According to Weidenreich, the 'cell-nests' of the bone-marrow 

 constitute the blood-forming tissue. These cell-nests are append- 

 ages to the venous capillaries, and the endothelium of the latter 

 is deficient in the region of the cell-nests. 



Venzlaff maintains that erythrocyte differentiation takes place 

 within the venous sinuses of the marrow of birds from lympho- 

 cytes which have passed out of the 'Leukoblastenhaufen' (cell- 

 nests of Weidenreich). The endothelium of the sinuses is defi- 

 cient in the region of the latter. 



For the spleen of the chick Danchakoff states: ''The spleen 

 during the second stage of development is characterized by the 

 development of a net of wide venous capillaries developed in the 

 mesenchymatous syncytium, by an intense granulopoiesis outside 

 the vessels and by a potential erythropoiesis within the vessels." 

 "The granulo- or leukopoiesis develops around the large venous 

 sinuses under conditions identical to those under which they 

 develop in the yolk-sac and in the bone marrow." 



For the spleen of the pig it has already been shown that early 

 erythropoiesis is largely extravascular. The first groups of eryth- 

 roblasts are usually at some distance from the sinuses in lacunae 

 of the dense mesenchyme. As the erythropoietic process becomes 

 more active the mesenchyme acquires a looser texture, owing to 

 the fact that many of its cells are cut off from the general syncy- 

 tium. This finally results in the formation of a loose network of 

 mesenchyme through which the erythroblasts are carried into 

 the sinuses. This is the structure of the pulp in embryos of 15 

 to 17 cm., and in these spleens the sinuses contain numerous ery- 

 throblasts. Some hemocytoblasts and nucleated red cells are 

 also seen in the sinuses of younger embryos, but intravascular 

 erythropoiesis is not very active until communication has been 

 established between the pulp lacunae and the sinuses. 



