304 GEO. A. THIEL AND HAL DOWNEY 



that the spleen of the embryo is a very active erythropoietic 

 organ. The beginning of this activity is seen in pigs of from 4 to 

 6 cm., although at this stage the process is limited to a few sharply 

 defined areas. The process gradually spreads in the older em- 

 bryos until in the 17.5-cm. stage it involves practically the entire 

 organ, excepting the region of condensed mesenchyme surround- 

 ing the developing arteries (fig. 8, 7.5 cm,). 



\ATien the first erythroblasts can be recognized (4 to 6 cm.) 

 there are a few venous sinuses present, and some of them con- 

 tain erythroblasts. But all of the erj^throblasts are not within 

 sinuses, as is claimed by Danchakoff for the chick. They occur 

 in small groups, many of which are at a considerable distance 

 from the nearest sinus. They are differentiated from similar 

 groups of hemocytoblasts located within lacunae in the dense 

 mesenchyme. Since the hemocytoblasts are derived from the 

 mesenchyme by the isolation of some of its cells from the general 

 syncytium, it is to be expected that the free cells will be con- 

 tained in cavities of the mesenchyme. Neighboring lacunae 

 containing free cells run together and produce the larger spaces 

 containing the first erythroblasts. These spaces are not sinuses, 

 for the latter are produced as splits in the mesenchyme indepen- 

 ent of the process of hemocytoblast formation, although a few 

 hemocytoblasts may be formed from the margins of the sinuses 

 after the latter have become established (fig. 4). The larger 

 spaces containing nests of erythroblasts may later connect with 

 sinuses, and hemocytoblasts entering the sinuses usually differ- 

 entiate into erythroblasts. For these reasons numerous sinuses 

 are encountered which contain er^ythroblasts in various stages of 

 differentiation. In many cases, however, the erythroblast nests 

 remain separated from the sinuses for some time. Frequently 

 mesenchyme cells are seen to pass between the individual cells of 

 a group, which is further proof that these cells are not located 

 within sinuses. It is quite possible that many of the larger 

 lacunae later function as sinuses. 



In the later stages (17 cm.) the erythroblasts do not show the 

 distinct grouping which is so characteristic of the earlier stages 

 of erythropoiesis. They are uniformly distributed throughout 



