DEVELOPMENT OF THE MAMMALIAN SPLEEN 327 



SUMMARY 



1. The spleen of the white rat, pig, and gopher is first seen as a 

 dense mass of mesenchymatous tissue in the left dorsolateral por- 

 tion of the mesentery. In the first stages of its development it 

 is impossible to make a sharp distinction between the coelomic 

 epithelium and the mesenchyme of the mesogastrium. Prolifera- 

 tive activity on the part of the epithelial cells may result in the 

 crowding of some of the daughter cells into the underlying mes- 

 enchyme. After the splenic rudiment is estabhshed (15-mm. 

 pig), the peritoneal cells are sharply separated from the mesen- 

 chyme by a distinct limiting membrane. 



2. The earliest splenic vessels are branches of the mesenteric 

 artery. These continue to bifurcate and form a network which 

 supplies the entire mesenteric tissue. This primitive capillary 

 system communicates with both arteries and veins. In 6- to 

 7-cm. pig embryos there is communication between the vascular 

 capillary network and the primitive splenic sinuses, and the open 

 circulation that is characteristic of the adult spleen is established. 



3. In the pig the blood-forming activit^" of the spleen begins 

 in embryos of from 3 to 4 cm. in length. Cells of the mesen- 

 chyme which bear no definite relation to vessels or sinuses become 

 more basophilic and lose their connection with the mesenchymal 

 syncytium. When free these cells become the large lymphocytes 

 which are typical of the early stages in the development of all 

 hematopoietic organs. 



4. Erythropoietic activity in the spleen is recognized first in 

 4- to 6-cm. pig embryos. Erythroblasts are seen in small groups 

 in lacunae that were formed by the first free cells as they w^ere 

 cut off from the mesenchymal syncytium. The first groups of 

 erythroblasts are usually at some distance from the sinuses, which 

 are produced as sphts in the mesenchyme independent of the 

 process of hemocytoblast formation. At a later stage, however, 

 a few hemocytoblasts which differentiate into erythrocytes may 

 be formed from the margins of sinuses. Erythropoiesis, w^hich is 

 largely extra- vascular, becomes very active in 15- to 17-cm. pig 

 embryos, converting the greater part of the organ into a pulp- 

 like structure. 



