DEVELOPMENT OF THE MAMMALIAN SPLEEN 307 



According to Danchakoff, the presence of the blood-plasma in 

 the venous sinuses is responsible for the exclusive intravascular 

 erythropoiesis in the bird's spleen. This may be the determining 

 factor even in the mammalian spleen, where the process is largely 

 extravascular, because the 'open' circulation is established in lim- 

 ited regions of the spleen soon after the first sinuses are formed. 

 This is seen even in a 6-cm. embryo in which small areas of the 

 spleen pulp are infiltrated with fully differentiated erythrocytes. 

 The mesenchyme of these areas forms a loose network, and it is 

 clear that the erythrocytes are not located within vessels. Since 

 the erythropoietic function of the organ is still in its earliest 

 stages, it is clear that the erythrocytes of these areas are not of 

 local origin. Plasma may diffuse throughout the mesenchyme 

 from these areas of infiltration, and this may influence the dif- 

 ferentiation of the hemocytoblasts. Under such conditions, the 

 distinction between intra- and extravascular erythropoiesis seems 

 to be of doubtful value. 



Few granulocytes are formed in the pig's spleen at any stage 

 of its development. However, a few mononuclear granulocytes 

 were seen in the spleens of 7.5-cm. and 13-cm. embryos. The 

 great majority of them are located in the marginal regions of the 

 organ where the mesenchyme is still very dense and where venous 

 sinuses and the 'open' circulation have not yet been established. 

 They are not grouped in 'nests' as are the erythroblasts and each 

 cell is located within a lacuna of the dense mesenchyme. The 

 cytoplasm of many of these cells is still very basophilic and the 

 nuclear structures indicate that the granules have differentiated 

 in cells which have but recently been cut off from the mesen- 

 chyme. Fine processes are sometimes seen to connect with the 

 surrounding mesenchyme, indicating that the granules may 

 appear before the cells are completely isolated. There is little 

 dense mesenchyme left in the spleen of the 13-cm. embryo, hence 

 the granulocytes are located immediately under the capsule. 



In the pig's spleen there is no evidence whatever for Schridde's 

 and Turk's claim that the granulocytes are derived from 'myelo- 

 blasts' which have their origin in the endothelium of the blood- 

 vessels and sinuses. Erythroblasts may be derived from the 



