THE ENDOCRINAL ORGANS 



46 = 



sented by the epithelial ventral termination of the infundibulum. The 

 anterior lobe is represented by the hypophysial duct, an ectoderm-Uned 

 tube, which opens in front of the mouth anteriorly and into the pharynx 

 posteriorly. Into this duct the paired olfactory pits open. The only 

 element of the myxinoid pituitary which is glandular is the intermediate 

 lobe, represented by clusters of cells lying between the infundibulum and 

 the hypophysial duct. These cells are proliferated from the ectoderm 

 of the hypophysial duct, just as in embryos of the higher vertebrates 

 the cells of the intermediate lobe are proliferated from the hypophysis 

 (Rathke's pouch). The scarcity of blood-vessels in the region of 



INFUNDIBULUM 



HYPOPHYSIAL DUCT 

 B. PE.TROMYZON 



C. HEPTANCHUS 



PARS TUBERALISV 



INFUNDIBULUM 



— ANTERIOR LOBE- 



POSTERIOR 



•-"^^ ANTERIOR LOBE- 



INFUNDIBULUM 



POSTERIOR LOBE 



0. RANA E. REPTILE F. HOMO 



Fig. 384. — A series of diagrams showing conditions in six different vertebrates, which 

 are beUeved to represent stages in the evolution of the pituitary gland. The complexity 

 of origin of this gland is correlated with the complexity of its endocrinal functions. The 

 posterior lobe is cross-hatched, intermediate lobe stippled, anterior (hypophysial) lobe 

 piebald, and the pars tuberalis solid black. (Redrawn from Oppel, after Stendell.) 



the intermediate lobe of the pituitary in cyclostomes makes it seem likely 

 that, if it has an endocrinal function in this group, its secretions are 

 poured into the infundibulum. Since the epithelium of the infundibulum 

 and of the hypophysial duct is not thickened but remains single-layered 

 in myxinoids, there is no evidence that these elements in this group have 

 an endocrinal function. Stendell, therefore, seems justified in the 

 conclusion that the intermediate lobe is the first part of the vertebrate 

 pituitary which is differentiated as an endocrinal organ. (Fig. 384) 



An advance in the evolution of the pituitary is found in Petromyzon. 

 In this animal, as in all higher vertebrates, connexion of the hypophysial 

 duct with the pharynx is lost and the organ ends blindly at its posterior 

 end. From it, however, are proliferated cells into the region between 

 the duct and the infundibulum. The cells which take a position next 



