224 Basic Structure of Vertebrates 



brates except cyclostomes and possibly .the bony fishes. They may 

 have an embryonic endocrinal function. 



Pituitary Gland 



The pituitary gland is located at the base of the brain just behind 

 the optic chiasma, where it is attached by a stalk to the lower end of 

 the infundibulum. Lodged in a cavity, the sella turcica, in the sphe- 

 noid bone, the mammalian pituitary is as safe from injury as possible 

 (Fig. 190). 



Like the suprarenals, the pituitary is formed of two elements of 

 diverse origin and function (Fig. 191). The chief parts are a larger 

 anterior and a smaller posterior lobe. The latter is partly surrounded 

 by the anterior lobe. Between the two major lobes are two smaller 

 glandular masses, the pars intermedia and the pars tuberalis. The 

 anterior lobe consists of epithelial cells forming cordlike masses among 

 which are numerous spaces filled with blood or lymph. In the pars 

 intermedia are some colloid-filled follicles resembling those of the 

 thyroid. The cells of the posterior lobe resemble certain non-nervous 

 ectodermal cells (neuroglia) of the brain. Among them are numerous 

 connective-tissue fibers. 



The pituitary gland has a double origin. The anterior lobe comes 

 from an ectodermal sac, "Rathke's pouch" (hypophysis), in the roof 

 of the mouth, while the posterior lobe is formed as an outgrowth of 

 the base of the diencephalon (Figs. 191, 192). The posterior end of 

 Rathke's pouch comes into contact with the infundibular outgrowth 

 from the base of the brain. As development proceeds, Rathke's pouch 

 loses connection with the ectoderm and breaks up into vesicles, the 

 cavities of which are remnants of the cavity of the pouch. The ventral 

 end of the infundibulum becomes thickened to form the posterior lobe. 

 The cells which later become differentiated as pars intermedia and 

 pars tuberalis are derived from the hypophyseal pouch. 



The pituitary gland occurs in all vertebrates. Its elements make 

 their first appearance in cyclostomes from whose hypophyseal pouch 

 (or "duct": Fig. 191) are proliferated cells which form an inter- 

 mediate lobe, while other clusters of cells produce hollow vesicles 

 which are believed to represent the beginning of a glandular anterior 

 lobe. The infundibular (posterior) lobe, however, is not definitely 

 differentiated. In sharklike fishes, the infundibular lobe of the gland 

 is only slightly indicated. Through the vertebrate series from fishes 

 to man, all three elements seen in cyclostomes are present. The anterior 

 lobe steadily increases in relative size while the intermediate lobe 

 shrinks. An increase takes place in size and differentiation of the pos- 

 terior lobe. The presence of colloidal material in the pituitary in all 



