DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA. 87 



lary substance of the suprarenal. The irregular polygonal 

 areas, in which the cells are embedded, are not unlike the 

 cell groups of the medullary substance, and might be easily 

 transformed into them. This, taken in connection with the 

 facts that this structure {p,p, figs. 9 and 10) is directly con- 

 tinuous with the medullary substance, and seems gradually 

 to pass into it, and that it is stained by bichromate of potash 

 seems to justify this conclusion. Its posterior extension 

 presents no objection to this view, as we have seen that in 

 the adult suprarenal the medullary substance is found 

 outside the organ in a corresponding region. 



The results at which I have arrived in the preceding pages 

 may be summed up as follows : 



(1) The suprarenal bodies in Mammalia are composed 

 of two parts — the cortical and the medullary — totally different 

 in their origin. 



(2) The cortical substance arises from the mesoblast. 



(3) The medullary substance is derived from the peri- 

 pheral part of the sympathetic system, and is at first 

 placed outside of the cortical substance, becoming trans- 

 ported into the middle of the suprarenal body in the course 

 of development. 



I may also call attention once more to the interesting 

 gradation of the structure of the suprarenals in different 

 groups of Vertebrata. In Elasmobranchs the two com- 

 ponents of the suprarenals are totally independent of each 

 other. The sympathetic part is found in successive sympa- 

 thetic ganglia, while the mesoblastic part is a median un- 

 paired body. In Reptilia the sympathetic part is no longer 

 bound up in sympathetic ganglia, but is closely applied 

 against the dorsal side of the mesoblastic part, which is 

 paired. In Aves the sympathetic part has found its way 

 inside of the mesoblastic part, but is as yet irregularly scat- 

 tered throughout the organ (Brunn). Finally, in Mam- 

 malia the sympathetic part is collected into one mass, and 

 occupies an area in the centre of the mesoblastic part, 

 although still showing its origin in its development. 



In conclusion, I wish to express my sincerest obligations to 

 Mr. Balfour, with whom the idea of the present investiga- 

 tion originated, for his uniform kindness in giving me his 

 valuable advice, and also for the facilities which he has 

 afforded me in his laboratory for pursuing my work. 



