THE SUPRARENAL SYSTEM 133 



mental stage with other heterogeneous tissues, such as the carotid 

 gland and the interrenal tissue. 



The development of the suprarenal represents a later chapter 

 in the history of both systems ; it is the last genetic event in which 

 a combination of interrenal and adrenal tissue takes place. It is 

 seen for the first time in the amphibia, where groups of character- 

 istic cells, released from the sympathetic ganglia, wander along 

 the wall of the aorta in a ventral and median direction, finally 

 reaching the interrenal bodies, which are already fully formed 

 and clearly defined. In the gymnophiona, these cell-groups place 

 themselves by the side of the interrenal cell-bodies, but in the 

 urodela and anura they become either partially or wholly inter- 

 mingled with them. The result is the formation, not of a supra- 

 renal, but of a number of small suprarenals. 



In reptiles and birds the process is analogous. At the begin- 

 ning, the interrenal system consists of isolated buds; at a later 

 period, sympathetic elements associate themselves with the meta- 

 morphosed and well-defined interrenal body and, in the course of 

 histogenetic development, this association becomes more intimate. 

 Almost the entire interrenal system is used up in the formation of 

 the suprarenal bodies, and the accessory interrenal bodies, which 

 are rare in these orders, are probably formed by secondary division 

 of the principal organ. 



In mammals, at the time when the interrenal system has be- 

 come clearly defined and histologically differentiated, which in 

 the human embryo is at about the fifth week, undifferentiated sym- 

 pathetic elements wander from the principal mass of the abdominal 

 plexuses to the cell-columns of the interrenal bodies. These cells 

 break through the capsule upon the median side and make their 

 way between the cell agglomerations and columns of the interrenal 

 tissue, at the same time keeping their structural relationship with 

 the abdominal sympathetic. In the external mass, a differentia- 

 tion between nerve-cells and chromaffine cells is soon perceptible; 

 but the cells which made their way into the interior remain for a 

 long time in their primordial state. This intrusion of sympathetic 

 elements is continued during the whole of foetal life, the interrenal 

 tissue becoming entirely interwoven with these elements, as seen 

 in the completed suprarenal of birds. It is not until later, when 

 the histological and cytological transformation of interrenal tissue 

 into the three-layered suprarenal cortex is complete, that the inter- 

 polated sympathetic elements become, in a topographical sense, the 

 suprarenal medulla. At about the beginning of the fourth month, 

 they acquire the chrome-brown colour which is their chief histo- 

 logical characteristic. That in mammals, as in the other higher 

 orders of vertebrates, the whole of the interrenal tissue is not used 

 up in the formation of the suprarenals, is shown by the fact that 

 accessory suprarenals frequently occur which contain no trace of 

 chromaffine tissue. These structures, which should rightly be 



