404 THE DEVELOPMENT OF THE CHICK 



The terminology does not, therefore, describe well the topo- 

 graphical arrangement of the components; it was derived from 

 the condition found in many mammals, the cortical cords of the 

 birds corresponding to the cortical substance, and the medullary 

 cords to the medullary substance of mammals. The medullary 

 cords are often called phseochrome or chromaffin tissue on account 

 of the specific reaction of the constituent cells to chromic acid, 

 and their supposed genetic relation to tissue of similar composition 

 and reaction found in the carotid glands and other organs asso- 

 ciated with the sympathetic system. 



The embryonic history has been the subject of numerous 

 investigations, and has proved a particularly difficult topic, if 

 we are to judge from the variety of views propounded. Thus 

 for instance it has been maintained at various times: (1) that 

 cortical and medullary cords have a common origin from the 

 mesenchyme; (2) that they have a common origin from the 

 peritoneal epithelium; (3) that the origin of the cortical and 

 medullary cords is absolutely distinct, the former being derived 

 from the sexual cords by way of the capsules of the renal cor- 

 puscles and the latter from the sympathetic ganglia; (4) that 

 their origin is distinct, but that the cortical cords are derived 

 from ingrowths of the peritoneum, and the medullary cords from 

 sympathetic ganglia. The first view may be said now to be 

 definitely abandoned, and no one has definitely advocated a 

 common epithelial origin since Janosik (1883). Thus it may 

 be regarded as well established that the two components have 

 diverse origins, and it seems to the writer that the fourth view 

 above is the best supported. (See Poll and Soulie.) The com- 

 parative embryological investigations strongly support this 

 view. 



Origin of the Cortical Cords. According to Soulie, the 

 cortical cords arise as proliferations of a special suprarenal zone 

 of the peritoneum adjacent to the anterior and dorsal part of 

 the germinal epithelium. This zone is distinguishable early on 

 the fourth day, and begins about half a millimeter behind the 

 glomeruli of the pronephros, extending about a milUmeter in a 

 caudal direction. Proliferations of the peritoneal epithelium are 

 formed in this zone, and soon become detached as groups of 

 epithelial cells lying in the mesenchyme between the anterior 

 end of the Wolffian body and the aorta. Such proliferation con- 



