EQUIVALENCE OF HEMATOPOIETIC ANLAGES. I. SPLEEN 270 



The intense development of the arterial vascularization begins 

 at this time and the spleen enters in the III phase of its develop- 

 ment, characterized by the appearance of follicles. A cursory 

 glance on spleens during development of arteries brings forth a 

 striking difference between the development of the veinous and 

 of the arterial vascularization. The arteries and their smaller 

 branches never appear as irregular splits limited by mesenchy- 

 mal cells. These vessels on the contrary always develop as reg- 

 ular narrow tubes (fig. 9, Art.c.) I did not intend to undertake a 

 special study of the vascularization of the spleen, therefore I did 

 not apply special methods of investigation for this purpose. 

 However, a thorough study of preparations may give some infor- 

 mation upon the development of the veinous and arterial vascu- 

 larization. The veinous sinuses and capillaries are local forma- 

 tions, the arteries and their branches seem to grow into the 

 spleen from outside and here ramify by budding (fig. 9). 



The arteries, growing into the pulpa like tissue of the spleen, 

 divide it in regions, which are soon further subdivided by smaller 

 arterial ramifications. The arteries lie first in the mesenchyme, 

 where numerous granulocytoblasts are present (fig. 9). The 

 mesenchymal cells continue to proliferate around the arteries. 

 The process of splitting off of lymphoid hemocytoblasts persists 

 in these regions, but their differentiation into granulocytoblasts 

 is suspended under conditions in which the mesenchymal cells 

 develop around the narrow arteries. These conditions do not 

 correspond to those which prevail around the thin walled veinous 

 sinuses. The arteries and their smaller branches finally become 

 surrounded by clear zones of mesenchymal cells (fig. 10), which 

 markedly contrast with the granuloblastic tissue. These zones 

 appear in preparations in the form of islands of mesenchymal 

 tissue, which fill up all the interstices between the arterial 

 vessels and the pulpa like tissue of the spleen anlage. These 

 islands are anlages of follicles. 



At the time of the intense development of the mesenchyme 

 around the arteries a new line of cell differentiation may be 

 traced in the spleen which soon will become predominant; this 

 is the differentiation of small lymphoctyes. The figures 12 



