EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 291 



The intense development of erythropoiesis at the expense of 

 lymphoid hemocytoblasts, which develop locally is another 

 striking evidence for the truth of the monogenetic conception 

 of the blood development and consequently for the polyvalency 

 of the mesenchymal cells in the spleen anlage. The same 

 mesenchymal cells, which normally would remain between the 

 vessels and either become a part of the reticular tissue or differ- 

 entiate into granular leucocytes now differentiate into red blood 

 cells after they are discharged into the vessels. 



Deficiencies in the development of vascularization and their ej 

 — The deficiencies in the vascularization of the spleen anlage 

 occur as results of the changes in the spleen incited by stimu- 

 lation of the stem cells. They can affect either the development 

 of the veinous net or the ingrowth and the distribution of arteries. 



The stimulation applied at the 7 to 8 day of incubation may 

 convert the whole spleen anlage or considerab e parts of it into 

 granuloblastic tissue. The sinuses develop thereby defectively 

 and considerable accumulations of granuloblastic tissue are often 

 provided merely with scant and narrow capillaries. The scarce 

 development of veinous sinuses is the natural sequence of the 

 excessive differentiation of granuloblastic tissue. The study of 

 the normal spleen development has shown, that the sinuses de- 

 velop locally as splits amidst the mesenchymal tissue. Since the 

 greatest part of mesenchyme may be transformed into accumu- 

 lations of free ameboid cells (fig. 20), the net of sinuses cannot 

 locally develop between the free cells. 



Defects in the development of veinous sinuses become them- 

 selves the source of interesting alterations in the spleen tissue. 

 The normal histogenesis leads to a gradual differentiation of 

 granular leucocytes at the expense of granuiocytoblasts. The 

 granulocytoblasts, though ameboid, do not migrate normally in 

 the vessels which evidently do not contain adequate chemiotactic 

 substances. The granular leucocytes, however, penetrate easily 

 into the veinous sinuses through thin walls (fig, 11 > Z) and 

 are drifted by the blood current away. Though the differentiation 

 of granular leucocytes is in a normal embryonic spleen continuous, 



