466 COMPARATIVE ANATOMY 



to the infundibulum form the intermediate lobe. Other clusters of 

 cells nearer the hypophysial duct produce hollow vesicles which are 

 believed to represent the beginning of a glandular anterior lobe. Since 

 the remainder of the embryonic hypophysis persists in the adult as a 

 blind pouch and is not, as in higher vertebrates, converted into anterior 

 lobe, it is evident that the anterior lobe of the pituitary of Petromyzon 

 is only partially homologous with that of the higher vertebrates. The 

 nervous lobe of the pituitary can hardly be said to exist in Petromyzon. 

 In this animal where the infundibulum is in contact with the cells of the 

 intermediate lobe, the epithelium is slightly thickened and the infundibu- 

 lum shows a well-marked hypophysial recess. 



The evolution of these elements which have their inception in cyclo- 

 stomes may be briefly summarized. As one passes through the verte- 

 brate 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 dififerentia- 

 tion of the posterior lobe. The presence of colloidal material in the 

 pituitary in all vertebrate groups justifies the assumption that the gland 

 has an endocrinal function throughout the series. In elasmobranchs 

 the nervous portion of the gland is only slightly indicated. The region 

 of the infundibulum to which the intermediate lobe is attached is thickened 

 in ganoids. The posterior lobe becomes more conspicuous as we pass 

 up the vertebrate series to mammals. 



According to the view just expressed, the evolution of the hypophysis 

 involves the metamorphosis of a tubular hypophysial duct into an endo- 

 crine organ. Another view is that the hypophysis was in the beginning a 

 gland which opened into the mouth, but for this opinion there seems to be 

 less evidence. 



