THE ENDOCRINAL ORGANS 337 



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

 end. From it, however, are proliferated cells which form the inter- 

 mediate 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 an anterior lobe, it is evident that the anterior lobe of the pituitary of 

 Petromyzon is only partially homologous with that of higher vertebrates. 

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



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

 stomes may be briefly summarized. In elasmobranchs the nervous portion 

 of the gland is only slightly indicated. 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 differentia- 

 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. 



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. 



