308 GEO. A. THIEL AND HAL DOWNEY 



walls of the sinuses at a time when the latter are merely splits in 

 the mesenchyme, but there is no evidence to indicate that cells 

 from this source might differentiate granules in their cytoplasm. 

 The free mesenchyme cells which differentiate into granulocytes 

 are far removed from vessels or sinuses, and hence it is not likely 

 that they bear any direct relationship to endothelial cells. 



The differentiation of a few erythroblasts from lymphoid cells 

 cut off from the marginal mesenchyme of the venous sinuses does 

 not confirm the claims of Schridde and Tiirk that erythroblasts, 

 as well as myeloblasts, are always derived from cells of the ves- 

 sel walls. The early sinuses have no walls, for they are merely 

 splits in the mesenchyme. 



At the 17-cm. stage the greater part of the spleen is composed 

 of free cells; the fixed mesenchymatous tissue has become greatly 

 reduced in amount, and it is evident that the majority of the lym- 

 phoid cells which are cut off from the mesenchyme differentiate 

 into erythrocytes. The greater part of the mesenchyme of the 

 pulp is thus used up in the process of erythrocyte production. 

 The remaining fixed tissue later differentiates into the loose net- 

 work of reticulum which forms the supporting framework of the 

 organ in the adult. 



Much of the condensed mesenchyme surrounding the develop- 

 ing arteries is also used up in the production of free cells during 

 the development of the lymphoid arterial sheaths. But in this 

 region the Ijmiphoid cells remain lymphoid and none of them dif- 

 ferentiate into erythrocytes or granulocytes. 



DEVELOPMENT OF THE ARTERIES AND LYMPHOID ARTERIAL 



SHEATHS 



The further differentiation of the vascular system leads to the 

 formation of distinct arteries, the development of which initiates 

 the formation of the 'white pulp' of the organ. The white pulp 

 is composed of the lymphoid sheaths which soon develop around 

 the arteries, and later distinct nodules are also added to it. The 

 arterial sheaths are at first composed of very dense mesenchyme, 

 as described by Danchakoff for the chick spleen. They are 

 surrounded everywhere by the loose pulp which, at this stage, 



