GROSS ANATOMY 31 



this time there occurs a proliferation of the epithelial cells 

 lying between the mesonephros and the root of the mesentery. 

 The buds of tissue thus formed, as found in a 12 mm. human 

 embryo, lie as a continuation of the suprarenal ridge. In the 

 20 mm. embryo, the sympatho-chromaphil cells begin to 

 migrate into the developing cortex along the line of the central 

 vein to form the medulla of the adrenal gland. It is not until 

 the embryo is 10 cms. in length that the migrating chromaphil 

 cells have reached the central vein and formed a true medulla. 

 In man this penetration of the epithelial anlage by the sympa- 

 thetic is accompanied by a special formation of folds and a 

 process of invagination. 376 On cross-section the cortex appears 

 invaginated deeply at certain points, especially along the 

 central vein, so that this vein is almost entirely surrounded by 

 cortical tissue, particularly in the adult. This peculiar process, 

 which is not limited to man, results in a more intimate contact 

 between medulla and cortex. 



Differentiation of the chromaphil cells in man begins in the 

 18 mm. embryo but is not complete until about birth. The 

 chromaphiloblasts (cells destined to become chromaphil tissue) 

 are larger in size and stain less intensely than the sympatho- 

 blasts. Their specific reaction with chromates appears at 

 this stage. Epinephrine does not appear in the human adrenal 

 until after birth. 393 



In mice the first appearance of the chromaphil reaction takes 

 place between the fourteenth and fifteenth day of develop- 

 ment. 317 At this time the future medullary cells are just 

 beginning to penetrate into the interrenal tissue leaving the 

 sympathetic ganglion anlage which at this time is closely applied 

 to the interrenal ridge. The sympatho-chromaphil mass at this 

 time is made up of cells of embryonic type. The differentiation 

 of the chromaphil cells is accompanied by the acquisition of 

 darker nuclei and the assumption of a chromaphil reaction. 

 The interrenal tissue at this time is surrounded by a layer of 

 fibroblasts. That the chromaphil reaction, as observed at an 



