282 GEO. A. THIEL AND HAL DOWNEY 



The material was embedded in paraffin and the sections 

 mounted in serial order. After examining a few embryos it was 

 found that the early splenic tissue was extremely dense and that 

 thin sections were highly essential for a cytological study of the 

 organ. For this reason all of the younger stages were cut at a 

 thickness of 3^. Various combinations of stains were employed, 

 depending upon the nature of the splenic tissue at its various 

 stages of differentiation. For a general stain Dominici's eosin- 

 orange G-toluidin blue method proved to be the most satisfac- 

 tory. Benda's haematoxylin, counterstained with Van Gieson's 

 picrofuchsin, was of special value in demonstrating the cyto- 

 plasmic processes of the early mesenchyme of the splenic rudi- 

 ment. The May-Giemsa mixture was used most extensively 

 for all the later embryonic stages. Ehrlich'striacid followed with 

 toluidin blue, and Ehrlich's staining mixture of induUn-aurantia- 

 eosin gave excellent material for a study of the granulocytes of 

 the organ. Weigert's elastic tissue stain proved very satisfactory 

 for the demonstration of the distribution of the elastic fibers sur- 

 rounding the early arteries, and Mallory's Thosphowolfram- 

 saiire' stain together with Krause's gold chloride, counterstained 

 with May-Giemsa, gave excellent material for a study of the 

 fibers within the cells of the early reticulum. Among the many 

 other stains were aqueous and alcoholic mixtures of haematoxylin, 

 Mallory's connective-tissue stain, and Pappenheim's methyl- 

 green pyronin. 



OBSERVATIONS 



Early splenic rudiment 



In embryos of white rat, gopher, and pig, the anmials used in 

 this study, the spleen is seen to develop in the dorsal mesogas- 

 trium at a level with the fundus portion of the stomach. When 

 first discernible as a distinct structure it appears as a dense mass of 

 tissue in the left dorsolateral portion or the mesentery, occupying 

 approximately one-half of the dorsolateral extent of that structure. 



Owing to the controversy regarding the derivation of the cells 

 of the rudiment, the study of the mesogastrium of younger em- 



