DEVELOPMENT OF THE MAMMALIAN SPLEEN 305 



the entire pulp and are located within the sinuses as well as in the 

 pulp strands between the sinuses. The majority of the hemocy- 

 toblasts and younger erythroblasts are outside of the sinuses. 



Similar conditions were seen by H. Fischer in the spleens of 

 human embryos, where hematopoiesis was very active in the veins 

 and venous sinuses of the pulp. According to M. B. Schmidt, on 

 the other hand, intravascular erythropoiesis in the venous sinuses 

 of the pulp from cells derived from the endothelium is of greatest 

 importance. 



According to Danchakoff, the lines of differentiation pursued 

 by hemocytoblasts depend entirely on extrinsic environmental 

 conditions. The relations of the primitive blood-cells to the 

 venous sinuses are of particular importance in this connection. In 

 the yolk-sac and bone-marrow of birds and reptiles and in the 

 spleen of birds those hemocytoblasts which pass into the larger 

 blood-channels are differentiated into erythrocytes, while those 

 outside of the vessels either differentiate into granulocytes or 

 remain as lymphoid wandering cells. Close proximity to the 

 sinuses favors granulocyte differentiation. Venzlaff also claims 

 that in the bone-marrow of birds erythrocyte development is 

 largely intravascular. 



Intravascular erythropoiesis in mammalian embryos, particu- 

 larly in the liver, is also claimed by Neumann, Schmidt, Kuborn, 

 Kostanecki, and Nageli, while van der Stricht, Maximow, 

 Schridde, Lobenhoffer, H. Fischer, and Mollier maintain that 

 erythropoiesis in the mammalian embryonic liver is chiefly ex- 

 travascular. According to Maximow and Weidenreich, the proc- 

 ess is also largely extravascular in the mammalian bone-marrow. 



For the embryonic marrow Maximow states that the develop- 

 ing cells lie in groups in close contact with the endothelium of the 

 vessels. In such places the endothelial membrane later loosens 

 up and acquires numerous openings. Through these openings 

 blood-plasma enters the tissue, loosens up the groups of develop- 

 ing erythrocytes and forces them into the vessels. Besides the 

 young denucleated erythrocytes a few of the riper normoblasts 

 and megaloblasts are carried into the circulation where they com- 

 plete their development in a normal manner. Lymphocytes are 



