DEVELOPMENT OF THE MAMMALIAN SPLEEN 283 



bryos which have not yet developed a splenic rudiment is of 

 special importance and interest. A 3-nim. gopher embryo 

 seemed to present favorable conditions for such a study. 



In this embryo the stomach is already formed and possesses a 

 short but distinctly developed mesentery. The latter is com- 

 posed of a loose mesenchymatous network covered on both sur- 

 faces by the visceral peritonemn consisting of several layers of 

 somewhat elongated cells rather loosely arranged. Within the 

 mesenchyme there are a few isolated free cells and capillaries con- 

 taining primitive erythrocytes. 



The structure of the coelomic epithehmn and its relation to 

 the underlying mesenchyme is of special interest, because many 

 authors have claimed that the mesenchyme in the region of the 

 future splenic rudiment is increased by the addition of cells from 

 the epitheUum. Toldt and Janosik claim that all of the cells of 

 the rudiment are derived from this source. 



In the embryo under discussion the coelomic epithelium ap- 

 pears more as a condensed region of the mesenchyme, rather than 

 as a distinct epithelium. The cells are closely approximated 

 and they are elongated somewhat in the direction vertical to the 

 surface of the mesentery. This condensation is two or three 

 cells deep, but the cells are irregularly placed and not arranged in 

 very distinct layers. There is no boundary line between them 

 and the mesenchyme, and the lower cells are directly continuous 

 with those of the mesenchyme, so that the whole structure appears 

 asa continuous syncytium. This is a general condition throughout 

 the mesentery and is not confined to the region of the future 

 spleen. With such an arrangement cells from the coelomic epi- 

 theUum could very readily pass into the mesench3ane. 



This close relationship between coelomic epithelium and mes- 

 enchyme was also described by Choronshitzky, Tonkoff, Laguesse, 

 and Mietens. Choronshitzky believed that both the endoderm 

 and the coelomic epithelium give off cells to the mesenchyme. 

 The coelomic epithelium is designated as a 'Keimepithel' for 

 the mesenchyme. Tonkoff noted continuity of epithelium and 

 mesenchyme just before the appearance of the splenic rudiment. 

 In later stages he found that the two were separated by a distinct 



