GROSS ANATOMY 29 



are thus ectodermal in origin. Although the chromaphil tissue 

 develops in intimate relation with the sympathetic nervous 

 system, the two are developmentally distinct in origin. The 

 differentiation of chromaphiloblasts from sympathoblasts be- 

 gins in the embryo but is not complete until late in gestation or 

 in some species after birth. When any cells in a given area of 

 the sympathoblasts differentiate, all do, and hence no inter- 

 mixture of chromaphil and sympathetic cells results despite 

 their ultimate intimate relations and common origin. 



The cortex of the adrenal and the interrenal tissue in general 

 are derivatives of the ventral portion of the coelomic epi- 

 thelium. They are thus mesodermal in origin. Embryo- 

 logically, therefore, the cortex and medulla are quite distinct 

 in origin. It is not until we reach the amphibia that the 

 union of the two anlage begins to form the compound organ 

 denoted as the adrenal. 



In the present section we shall outline briefly the embryo- 

 logical development of the adrenal and its homologues in the 

 elasmobranchs, amphibia, birds, and mammals. For more 

 detailed descriptions the excellent monographs of Balfour, 28 

 Soulie\ 581 and Poll 608 should be consulted. 



Elasmobranchs. The development of the interrenal in the 

 dogfish (Scylliwn) has been studied by Poll. 508 In this elasmo- 

 branch the interrenal first appears in the 7 mm. embryo as a 

 number of irregularly distributed thickenings of the splanchnic 

 mesoderm, ventral to the dorsal aorta, and extending from the 

 pronephros as far back as the cloaca. These rudiments meet 

 and become continuous taking the form, in the 10 mm. embryo, 

 of a cellular rod lying under the dorsal aorta. In the 16 to 28 

 mm. embryo, the interrenal body gradually becomes separated 

 from the coelomic epithelium and the nephrotomes along 

 which it had previously been in apposition. Only the posterior 

 half of the rudiments take part in the above described fusion 

 to form the interrenal. The anterior rudiments atrophy or 

 develop into small accessory bodies. 



The chromaphil bodies of Scyllium develop at a much later 



