DEVELOPMENT OF THE MAMMALIAN SPLEEN 309 



consists largely of free cells. Later this dense mesenchyme is 

 resolved into a very loose network, the meshes of which are 

 packed with lymphocytes. Most of these free lymphoid cells are 

 'small lymphocytes/ and in these lymphoid sheaths and in the 

 earl}^ follicles it is not difficult to w^ork out their origin. 



Figure 7 is a cross-section of a typical artery and its lymphoid 

 sheath in a 30-cm. embryo. The vessel is surrounded by a single 

 layer of smooth muscle fibers which lie directly underneath the 

 endothelium of the vessel. When Weigert's elastic tissue strain 

 is employed, it is found that elastic tissue fibers, which extend 

 parallel with the artery, lie between the endothelial and muscle 

 cells, forming the beginning of the inner elastic membrane of the 

 arterial wall. In this figure but one endothelial nucleus is shown, 

 which extends well into the lumen of the vessel. The cytoplas- 

 mic processes that fringe the remaining portion of the lumen rep- 

 resent sections of the cell bodies of neighboring endothelial cells, 

 the nuclei of which were in a different plane than the section here 

 figured. 



The reticulum surrounding the muscular coat of the artery is of 

 particular interest; its cells possess long processes which form a 

 network, in the meshes of which numerous small lymphocytes are 

 located. At the outer border of this early lymphoid sheath, the 

 reticular cells are crowded into parallel bands that lie in concen- 

 tric order about the artery. 



A great variation is seen in the structure of the reticular nuclei. 

 The cell a in the upper part of the figure possesses a nucleus iden- 

 tical with those of the reticulum of the pulp portion of the organ. 

 It is a large oval structure with a distinct eccentrically placed 

 nucleolus and a very small amount of chromatin distributed in 

 the form of a fine network. The nuclear membrane is very pro- 

 nounced in contrast to those of the smaller reticular cells. The 

 cell at h, although connected by cytoplasmic processes with the 

 reticular cell already described, shows very marked differences. 

 Its nucleus is scarcely half as large as those of* the reticular cells 

 of the pulp. No distinct nucleolus is present, and the small 

 amount of chromatin is uniformly distributed throughout the 

 nucleus, but the nuclear membrane is quite indistinct. Many in- 

 termediate stages can be found between these two types of cells. 



