260 



THE ENDOCRINE GLANDS 



texture, and dark red in color. The two zones ha\e an entirely 

 different embryonic origin. (Fig. 161.) 



The cortex develops as a series of buds from the anterior third 

 of the Wolffian body. The cells of the medulla originate from cells 

 related to those of the coeliac plexus of the sympathetic system and 

 form epithelioid groups. As development proceeds, the latter mass 

 of cells becomes invested with those of the cortical region. 



In fishes the medullary cords exist as small masses, each adjacent 

 to a sympathetic ganglion and arranged segmentally alongside the 



Fig. 161. — Photograph of a longitudinal section through the adrenal of the wood- 

 chuck. A connective-tissue sheath surrounds the entire organ. The darker cortex 

 surrounds the central lighter medulla, except at the hilus on the right of the figure. 

 Below are sections of blood vessels. 



elongated kidney, while in between the kidneys are two long masses 

 of tissue homologous with the mammalian cortex. In am])hibians 

 there is a closer association between the cortex portion and the 

 medulla. In reptiles the two are in contact, in birds they are inter- 

 mixed, and in mammals the cortex surrounds the medulla. 



Mammalian Adrenal.— Each gland in the case of mammals is 

 surrounded by a connective tissue capsule, which supports arteries, 

 veins, capillaries, and lymphatics. Strands of connective tissue 

 extend in between the epithelioid cells of the cortex into the medulla, 

 supporting a reticular tissue network and ca])illaries. 



The Cortex.-— Three zones of e])ithell()id cells may be distinguished 

 in the cortex. (Fig. 162.) Immediately beneath the capsul(> is a 



