292 GEO, A. THIEL AND HAL DOWNEY 



days) "the arteries and their branches seem to grow into the 

 spleen from the outside and here ramify by budding." The 

 veins and sinuses are local formations, whereas the arteries, 

 which develop as regular narrow tubules, grow into the pulp- 

 like tissue of the organ and there continue to divide and subdi- 

 vide into numerous ramifications. 



A comparison of these findings with observations on serial sec- 

 tions of early mammahan embryos, together with the details of 

 the differentiation that accompanies the early vascular transfor- 

 mations within the organ, will be discussed in the following section 

 of this paper. 



In order to determine the structure and distribution of the 

 early vessels of the spleen, a study was made of serial sections of 

 pig embryos from 1 to 6 cm. in length. In a 6-cm. embryo, 

 branches of the splenic artery exhibit sufficient differentiation 

 to facilitate their identification. In all embryos under that 

 size, the only vascular elements in evidence are liarrow, irregular, 

 and branching capillaries through which nucleated erythrocytes 

 make their way through the splenic rudiment. These primitive 

 vessels present a distinct endothelial lining, the . cytoplasm of 

 which can be easil}^ recognized because of its stainmg reaction 

 which is more intense than that of the splenic mesenchjone. 

 There are no instances of large unhned sinuses in the organ dur- 

 ing the first five weeks of embryonic life. 



As earl5^ as a 12-mm. embryo it is possible to trace the mesen- 

 teric artery along the midventral margin of the dorsal mesogas- 

 trium. At various points along its com'se tiny capillary branches 

 are given off that penetrate the mesenchyme of the early splenic 

 rudhnent. These continue to give off branches that form a net- 

 work which supplies the entire mesenteric tissue. This prunitive 

 capillary system persists until the embryo has reached a length of 

 4 to 6 cm. 



In the chick Danchakoff found none of these early capillaries. 

 The venous sinuses, which soon commimicated with the intes- 

 tinal veins, but not with the arteries, were the first vessels to 

 appear. In the pig, however, the first capillaries communicate 

 with both the arteries and veins. 



