294 GEO. A. THIEL AND HAL DOWNEY 



large lymphocytes (hemocytoblasts of Danchakoff) which become 

 so nmnerous in the later stages. Numerous examples of thi& 

 cutting-off process in this and later stages make it quite evident 

 that the mesenchyme bordering these primitive sinuses is very 

 active in the process of furnishing free cells to the sinuses. At 

 the same time these cells assume the characters of primitive lym- 

 phocytes or 'hemocytoblasts' which later function as the parent 

 cells of the erythroblasts which differentiate within the sinuses. 

 The process is similar to that described by Danchakoff in the 

 developing chick spleen, and by Downey and Weidenreich and 

 many others for the lymph sinuses of lymph nodes of adult 

 mammals. . 



In 6- to 7-cm. embryos a connection between the vascular sys- 

 tem and the primitive splenic sinuses is established, as is shown 

 by the sudden appearance of the fully differentiated erythrocytes 

 within the sinuses. This vascular transformation begins in the 

 central part of the organ and is at first distinctly localized. In 

 an embryo of 7.5 cm. large areas of the splenic tissue are infil- 

 trated with erythrocytes, but the regions are sharply cut off from 

 the remaining reticulum of the organ. They soon spread toward 

 the periphery, so that in an 8 cm. embryo only a narrow rim of 

 dense mesenchyme remains. 



When intense erythropoiesis is estabhshed (10 to 12 cm.), the 

 sinuses become greatly enlarged and are filled with differentiating 

 erythroblasts, many of the parent cells of which are derived from 

 the cells that border the lumen. Some of the border cells become 

 elongated and assume the characteristics of a flattened endothe- 

 lium. The walls of the early sinuses are extremely delicate^ 

 being composed of a single layer of flattened reticular cells (text 

 fig. A), between which a direct communication with the early 

 splenic pulp is retained. Through these meshes numerous blood- 

 cells migrate and are conveyed from the sinuses to the veins of the 

 organ. 



Text figm-es A and B are sections of sinuses from a 30-cm. em- 

 bryo. In figure A, two large cells are migrating from the splenic 

 pulp into an enlarged sinus. The foremost of the cells is a normo- 

 blast that already possess considerable hemoglobin and shows a 



