328 GEO. A. THIEL AND HAL DOWNEY 



5. The extensive extravascular erythropoietic activity may be 

 influenced by environmental factors that are introduced by the 

 early establishment of an 'open circulation' in the embryonic 

 spleen. 



6. Granulopoiesis is very limited in the pig's spleen at any 

 stage of its development. A few mononuclear granulocytes are 

 developed in the marginal regions of the organ from cells that are 

 cut off from the mesenchyme. There is no evidence that granu- 

 locytes are derived from 'myeloblasts' which have their origin 

 in the endothehum. 



7. The differentiation of 'white pulp' in the spleen is initiated 

 at the time of the formation of distinct arteries (6-cm. pig). 

 A lymphoid sheath is developed around the arteries. This arte- 

 rial sheath is at first composed of very dense mesenchyme which 

 later is resolved into a loose network, the meshes of which are 

 packed with small lymphocytes. The great majority of these 

 lymphocytes are cut off from the mesenchyme, and they differen- 

 tiate into typical small lymphocytes without passing through 

 the hemocytoblast stage. 



8. Although the lymphoid or follicular portion of the spleen 

 appears comparatively late in the development of the organ and 

 is confined to the region of the arteries, it cannot be regarded as 

 a tissue which is foreign to and of different origin from that of 

 the red pulp, for its mesenchyme is derived from and is continu- 

 ous with that of the pulp, and its free cells migrate into the pulp. 



9. Large lymphocytes are rare in the Ijrmphoid portion of the 

 spleens of embryos. Their appearance in postnatal animals is 

 associated with hypertrophy of the reticulum in the marginal 

 portions of the lymphoid arterial sheaths or in the germ centers 

 of developing foUicles. Most of the first large lymphocytes are 

 derived from this hypertrophied reticulum, but others are the 

 result of growth of small lymphocytes. Large lymphocytes soon 

 appear in all parts of the follicles and lymphoid sheaths and 

 many of them migrate into the pulp. 



