DEVELOPMENT OF THE MAMMALIAN SPLEEN 299 



In a 20-cm. embryo, the structure of the capsule has become 

 more distinct. Weidenreich is inchned to think of the cells that 

 make up the capsule as transformed endothelial cells. However, 

 by making a study of their development in a closely graded series 

 of embryos, the capsule cells are found to be cells of the general 

 mesenchyme that have drawn in their processes and become 

 more compact. 



The fact that they are not endothelium can be readily demon- 

 strated by applying Mallory's phosphotungstic-acid stain to- 

 gether with Krause's gold chloride for reticular fibers. This 

 method brings out the cytoplasmic processes of the reticular cells 

 and also the reticular fibers that extend for long distances from 

 cells of the capsules through the reticular cells of the pulp. 

 These fibers are not found in the endothelium that lines the vessel 

 of the capsule, nor is the same intimate connection between cyto- 

 plasmic processes in evidence. 



A great many erythrocytes are found in the walls of the cap- 

 sules. These red blood-cells pass through the porous wall of the 

 arterial capillary into the unlined spaces between the capsule 

 cells, and from there make their way into the meshes of the reticu- 

 lum of the pulp. 



The presence of a great many contorted nuclei within the cap- 

 sules of adult spleens has led many workers to believe that they 

 represent the presence of numerous leucocytes. A histogenic 

 study, however, indicates that by far the greater number are the 

 nuclei of the capsular cells. 



In the spleen of adult pig a band of fibrous tissue separates the 

 capsule from the splenic pulp, but in the embryo this sharp line 

 of demarcation is not present. 



THE ORIGIN AND DIFFERENTIATION OF THE FIRST BLOOD-CELLS 



In the pig the blood-forming activity of the spleen begins in 

 embryos of from 3 to 4 cm. in length, but the process of differen- 

 tiation along this line is subject to considerable variation in differ- 

 ent embryos of this size. In 3- to 3.5-cm. embryos a few isolated 

 fields are usually encountered where cells of the mesenchyme are 

 becoming more basophilic and are beginning to lose their connec- 



