296 GEO. A. THIEL AND HAL DOWNEY 



normal accompanying increase in the amoimt of chromatin mate- 

 rial in its nucleus. The cell to its left is a typical lymphoid hemo- 

 cytoblast with a large open nucleus, possessing a distinct nucleolus 

 and but a small amount of chromatin. Its cytoplasm is strongly 

 basophilic. Figure B shows a cell of the same tj-pe. 



By comparing these two figui-es, a clearer conception of the 

 structure of the wall of the sinuses can be obtained. In text 

 figure A the wall is seen to be continuous with the processes of the 

 neighboring reticular cells, so that an open path with the meshes 

 of the pulp is established. In text figure B a tiny cytoplasmic 

 thread, one of the torn ends of which extends over the cytoplasm 

 of the migrating cell, marks the presence of a continuous wall at 

 a plane slightly above the free cell, showing that the cell is either 

 penetrating the wall directly or is passing through a mesh in a 

 plane that is slighth' below the section here figured. 



During the de\'elopment of the organ the structure of the ven- 

 ous sinuses remains unchanged. Text figure C Avas drawn from 

 a preparation of the spleen of adult pig and it represents a portion 

 of a sinus with some of the sui-roimding pulp. It is evident that 

 the wall of the sinus is composed of a close network of reticulum 

 which is continuous with that of the surrounding pulp. The 

 sinuses are never lined with endothelimn; they are merely irregu- 

 lar channels through the reticulum, in the adult as well as in the 

 embrA'o. Cells from the pulp can readily make their way into 

 the sinus through the nmiierous openings which occur in the 

 reticular wall, one of which is shown in the figure. 



The terminations of the arterial capillaries in the pulp were 

 not studied, but the structure of the arterial capsules in 12 to 17 

 cm. pig embryos shows that blood-cells from the encapsulated 

 arterioles can readily make their way into the pulp. At this 

 stage the arterial capsules are already established and a striking 

 path of communication between artery and pulp becomes evi- 

 dent. This path lies through the unlined spaces of the tissue 

 of the early capsules, from which numerous channels radiate into 

 the meshes of the pulp. And, as has been shown above, many 

 connections between these meshes and the splenic sinuses are 

 maintained. Thus an open circulation is established before the 

 middle of embi'vonic life. 



