316 GEO. A. THIEL AND HAL DOWNEY 



the endothelium of the vessel taking no part in the process. It 

 might, of course, be assumed that the dense mesenchyme sur- 

 rounding the artery constitutes a part of the vessel wall, and in 

 that case the free lymphocytes liberated by the mesenchyme 

 would be derived from cells of the vessel wall — Schridde's 'Bkit- 

 gefasswandzellen.' However, according to the theory, lympho- 

 cytes would come only from the walls of lymph-vessels, while 

 the blood-vessel-wall cells would furnish only myeloblasts and 

 erythrocytes. In the developing spleen it is very clear that the 

 follicles and lymphoid sheaths are in no way associated with 

 lymph-vessels. They are developing about the arteries, and 

 lymph- vessels are not present: hence the facts do not conform to 

 the theorj^, even though we count the condensed mesenchyme 

 about the arteries as a part of the vessel wall. 



The endothelium of well-established vessels of the spleen ap- 

 parently plays no part in the formation of fixed or free cells, or 

 of 'adventitial' cells, about the vessel wall, and hence Herzog's 

 claim that the adventitial cells derived from the endothelium of 

 growing capillaries in experimental material give rise to numer- 

 ous small lymphocytes with dark nuclei does not apply to the 

 early spleen. 



RELATIONS BETWEEN PULP AND FOLLICULAR TISSUE 



The relationship of the spleen pulp to the malphigian bodies 

 is a question over which there has been much discussion among 

 hematologists. Because the early spleen is composed entirely of 

 'pulp,' and the small lymphocytes are never numerous until they 

 appear later in the adventitia of the arteries, a region which con- 

 tinues to furnish most of the small lymphocytes of the organ, 

 and because myeloid metaplasia is confined to the pulp, many 

 authors have believed that pulp and follicles are independent 

 in origin and function and that they are more or less antagonistic 

 to one another. 



Since the question is discussed in detail in the paper by Weiden- 

 reich and Downey, there is no necessity for reviewing the exten- 

 sive literature here. However, there are a few points regarding 

 the origin of pulp and follicles in the mammalian embryo which 

 need further consideration. 



