DEVELOPMENT OF THE MAMMALIAN SPLEEN 285 



nuclei are large and oval in shape, possessing a distinct nuclear 

 membrane and, in most cases, two well-marked nucleoli. The 

 remaining chromatic material is distributed in the form of a 

 few small, irregular granules. 



The cells shown at E are undoubtedly epithelial cells, but from 

 their position one would conclude that they are being crowded 

 into the mesenchyme. To the right and left of the figure the 

 epithelium is more clearly separated from the mesenchyme, and 

 one can readily distinguish the location of the limiting membrane 

 which separates the two tissues during later embryonic life. 



The marked changes that take place in the structure of the 

 peritoneal epithelium can be seen by comparing figures 1 and 2. 

 In figure 2, which is taken from a 15-mm. pig, the peritoneal cells 

 are not only sharply separated from the mesenchyme, but they 

 have undergone a marked differentiation that distinguishes them 

 from the characteristic mesenchyme cells. The greatest varia- 

 tion is seen in the nuclear structure. In figure 2 the epithelial 

 nuclei are almost uniformly round structures. The nuclear mem- 

 brane is still quite distinct, but the chromatin is distributed in 

 finely granular form throughout the whole nucleus. The nucleoh, 

 when present, are not as sharply defined as in the mesenchyme 

 cells. The cytoplasm of the two types of cells shows the same 

 staining reaction, but cell walls become quite evident between 

 adjoining epithelial cells, and at their base a distinct limiting 

 membrane, which is a du'ect modification of the basal portion 

 of their cytoplasm, is developed as further evidence of differen- 

 tiation. The membrane can be distinctly demonstrated with 

 either aqueous or alcoholic mixtures of hematoxylin and can also 

 be clearly recognized in material that has been stained with 

 Dominici's eosin-orange G and toluidin blue. 



The above findings, together with those of other workers, indi- 

 cate that in the early stages, before the splenic rudiment is 

 formed, it is impossible to make a sharp distinction between the 

 coelomic epithelium and the mesenchyme of the mesogastrium. 

 The cells of both are quite similar in structure (7.5-mm. pig) and 

 continuity of their processes is well established. Proliferative 

 activity on the part of the epithelial cells results in the crowding 



