286 GEO. A. THIEL AND HAL DOWNEY 



of some of the daughter cells into the underlying tissue where 

 they become a part of the general mesenchyme. After the 

 splenic rudiment has been established (15-mm. pig) the cells of 

 the epithelium have differentiated to such an extent that they 

 no longer resemble those of the mesenchyme. At this stage the 

 mesenchyme is separated from the epitheUum by a sharp line 

 and epithelial cells are no longer added to it. This corresponds 

 exactly to what was observed by Laguesse and Tonkoff. The 

 coelomic epithelium is, therefore, concerned only indirectly with 

 the splenic rudiment through the mesenchyme which by growth 

 and differentiation produces the rudiment. 



The first sign of a distinct differentiation into splenic tissue 

 was seen in an 8-mm. gopher embryo. In embryos of this size, 

 the rudiment is more easily distinguished by its position than by 

 the character of its cellular constituents. Its cells appear the 

 same as those of the surrounding mesenchyme, but the tissue is 

 much denser, being composed of mesenchymal cells with short 

 anastomosing processes which form a dense and compact syncy- 

 tial mass. This early condensation does not extend across the 

 entire mesentery, but is confined to its dorsal portion, near the 

 attachment or junction of the mesentery with the stomach wall. 

 There is no loose mesenchyme between the rudiment and the 

 coelomic epithelium. 



Some authors (Tonkoff, Choronshitzky, Laguesse, Kraatz, 

 Kolhnan, Minot) noted this early condensation of the mesen- 

 chyme, but others (Gray, Gotte, Mietens, Toldt, Maurer, v. 

 Kupffer, Janosik) did not recognize the rudiment until it was 

 composed of many free cells which were considered to have been 

 derived from the visceral coelomic epithehum, from endoderm, 

 or from primitive wandering cells, the mesenchyme serving merely 

 as a supporting tissue for these free cells. 



Danchakoff found the same conditions in the first stage of 

 splenic development in the chick. She, however, denies the pos- 

 sibility of coelomic epithelium playing a part in the formation 

 of the rudiment of the organ. She states that the ''development 

 of the spleen at the expense of the mesenchymal cells without 

 any relation to the endoderm nor the coelomic epithelium may be 



