53^ 



CHORDATE ANATOMY 



development proceeds, Rathke's pouch loses connexion with the ectoderm 

 and breaks up into vesicles, the cavities of which are remnants of the pouch 

 cavity. The ventral end of the infundibulum becomes thickened as the 

 anlage of the posterior lobe. The cells which later become differentiated 

 as pars intermedia and pars tuberalis are derived from the anlage of the 

 anterior lobe. 



The pituitary gland occurs only in vertebrates. The elements of the 

 pituitary make their first appearance in cyclostomes. In myxinoids 

 (Myxine, Bdellostoma) the neural (posterior) lobe is represented by the 

 epithelial ventral termination of the infundibulum. The anterior lobe is 

 represented by the hypophysial duct, an ectoderm-lined tube which opens 



' INFUNDIBULUM-. 



INTERMEDIATE LOBE 



Z.- PHARYNGEAL 

 ^ APERTURE 



ANTERIOR LOBE-'' 



INFUNDIBULUM 



'HYPOPHYSIAL DUCT 



B. PETROMYZON 



INTERMEDIATE 



C. HEPTANCHUS ^^^ 



(POSTERIOR LOBE INFUNDIBULUM 



INTERMEDIATE 



— ANTERIOR LOBE — 



PARS TUBERALISC 

 POSTERIOR 

 ^^ ANTERIOR LOBE 



INFUNDIBULUM 



POSTERIOR LOBE' 



D. RAMA E. REPTILE F. HOMO 



Fig. 298. — A series of diagrams showing conditions in six different vertebrates, which 

 are believed 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.) 



in front of the mouth anteriorly and into the pharynx posteriorly. (Fig. 

 298, .1) 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 hypophysial duct. 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. 298) 



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

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



