64 E. V. COWDKY 



the diapkragpn, reaching almost the full length of the body cavity. Later, 

 by a process of degeneration of some of the tissue and arrest of develop- 

 ment, while the remainder of the body continues to develop, the glands 

 come to have the relatively small size of their adult state. 



Origin of the Medulla. The cells making up the medulla are derived 

 from the primitive nerve tissue. At an early stage of development a mass 

 is separated from the lateral wall of the neural plate to form the neural 

 crest. From this mass cells migrate to the neighborhood of the aorta. 

 By the time the embryo has reached a length of seven millimeters the 

 migration is well under way. At nine millimeters the ganglionated cord 

 and the sympathetic nerve plexuses of the adult can be recognized. At 

 sixteen millimeters the outlying visceral ganglia have been established. 

 In close association with these structures are the cells which give rise to the 

 chromaphil tissues. Indeed, in the earlier stages, the two sorts of cells 

 are indistinguishable. They form together the sympathochromaphil 

 tissue. Later, as the two types become recognizable, they form the sym- 

 pathoblasts and the chromaffinoblasts. The chromaphil bodies develop 

 early, before the sympathetic cells proper, or the medulla of the adrenals 

 can be differentiated. When the embryo has reached a length of nineteen 

 millimeters the chromaphil cells have begun to penetrate well into the 

 cortical mass described in the preceding paragraph. At nine and a half 

 centimeters the chromaphil cells appear as islands scattered through the 

 cortical tissue, but some of the cells have already reached the central vein 

 and formed the nucleus of the medulla. From this stage on, development 

 consists of further penetration of the chromaphil cells to join the central 

 mass. The process is not entirely completed at the time of birth. 



Appearance of Hie Chromaphil Reaction. It is not until between the 

 third and the fourth month that the medullary substance acquires its 

 chromaphil reaction and may therefore be considered functional. That 

 such reaction is evidence of secretory potential, is shown by the observa- 

 tion that in the fully formed gland the coloration disappears as the epi- 

 nephrin is exhausted. ^Moreover, Lewis has been able to detect epinephrin 

 in extracts of the suprarenals of full term fetuses, 



Differentiation of Cortex. The differentiation of the cortex takes 

 place very slowly. According to Zuckerkandl, "the zona reticularis may 

 ho recognized in an embryo of fourteen and a half millimeters." Elliott 

 and Armour consider the enormous relative size of the suprarenals in 

 fetuses to be due to the high development of a portion of the cortex adjacent 

 to the medulla, which they call the boundary zone. The cells in the boun- 

 dary zone are devoid of lipoid at this stage of development. The lipoid is 

 restricted to a very thin layer of cells near the surface of the cortex. 

 Later on, the cells of the boundary zone accumulate fat and become typical 

 zona glomerulosa between the second and third years. 



