DEVELOPMENT OF THE MAMMALIAN SPLEEN 287 



regarded as a well founded fact." Such an interpretation may be 

 'well-founded' for lower vertebrate forms, but in mammals, one 

 cannot disregard the small but strikingly evident role that the 

 peritoneum plays in early embryonic life in adding its prolifera- 

 tive products to the underlying tissue. 



In neither of the types of embryos under observation was there 

 anything seen that would indicate the possibility of endodermal 

 cells from the intestinal epitheUum or from the early pancreatic 

 tubules making their way to the splenic rudiment. The immedi- 

 ate proximity of spleen and pancreas in the embryo was, no 

 doubt, a factor in inducing the earlier workers (Schenk, Woit, 

 Glas, and others) to seek for a genetic relationship between the 

 two organs. 



As stated above, the early spleen is characterized by its dense 

 mesenchymal syncytium. Due to intense cell proliferation, it 

 continues to develop until it acquires a length of 2 to 3 mm. in a 

 15-mm. pig embryo. In embryos of this stage there is but little 

 differentiation within the splenic tissue. 



THE VASCULARIZATION OF THE SPLEEN 

 1. General 



The question of the vascularization of the primitive spleen has 

 received very little attention, as most of the earlier workers con- 

 fined their observations to the fundamental problems concerning 

 the differentiation of the various germ layers and the part they 

 played in the formation of the different organs of the viscera. 

 It was not until hematologists undertook an explanation of 

 the relation of the spleen to the blood and lymphatic systems 

 that a more definite conception of the splenic circulation was 

 established. 



Before attempting a discussion of the splenic circulation of 

 the embryo, a survey of the different opinions in regard to the 

 vascularization of the adult organ may be permitted. This 

 question, which has been given much attention, centers upon the 

 relation of the so-called splenic sinuses or 'Venose Kapillaren' to 

 the rest of the circulatory bed. The presence of the cellular ele- 



