142 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



runner of the central nervous system was still a portion of the outer 

 surface of the body, we do not, in the present writer's opinion, 

 appear to be justified iii connecting up the eye of typical Verte- 

 brates with the " eyes " of Amphioxus or of Tunicates. It seems 

 more probable that the eyes of these highly specialized creatures are 

 organs which have developed independently within their own groups. 



Pituitaky Body. — To be included amongst the derivatives of the 

 ectoderm is that enigmatical organ the pituitary body (" Hypophysis 

 cerebri "). This — the " anterior lobe " of the pituitary body in 

 mammalian anatomy — arises normally as an ingrowing pocket of 

 ectoderm on the ventral side of the head, situated as a rule close to 

 the hinder limit of the stomodaeum but in the case of the Cyclo- 

 stomes just outside its anterior boundary. This pituitary involution 

 extends inwards beneath the infundibulum. In the Cyclostomata it 

 retains its original form of a tube communicating with the exterior, 

 but in the gnathostomatous Vertebrates its outer end becomes gradu- 

 ally constricted into a narrow duct which in the great majority of 

 cases becomes finally obliterated, so that the organ now forms a 

 closed sac lying immediately underneath the infundibulum. The 

 wall of the sac sprouts out into numerous tubular projections between 

 which develops highly vascular mesenchyme, providing the rich 

 blood supply necessary to the definitive function of the organ as a 

 ductless gland. 



As regards variations in the development of the pituitary involu- 

 tion : it may arise as a solid ingrowth of ectoderm (Lung-fishes, Am- 

 phibians) ; it may be two-lobed (Teleosts) or three-lobed (Lacertilia) 

 in its early stages ; its external opening may become secondarily dis- 

 placed up on to the dorsal side of the head (Lampreys) ; its inner 

 end may come to open secondarily into the pharynx (Myxinoids). 

 As already indicated the wall cf the infundibulum in the Gnatho- 

 stomata comes into intimate relation with the pituitary body in the 

 restricted sense, forming the so-called posterior lobe, or cerebral 

 portion, or nervous portion, of the pituitary body. 



LITERATURE 



Agar. Anat. Anzeiger, xxviii, 1906. 

 Agar. Anat. Anzeiger, xxxv, 1910. 

 Assheton. Quart. Journ. Mier. Sci., xxxiv, 1892. 

 Assheton. Quart. Journ. Mier. Sci., xxxviii, 1896. 

 Balfour. Phil. Trans. Roy. Soc, elxvi, 1876. 



Balfour. Monograph on the development of Elasmobranch Fishes. London, 1878. 

 Bidder und Kupffer. Untersuchungen iiber die Textur des Riiekenraarks u'nd die 

 Entwickelung seiner Formelemente. Leipzig, 1857. 



Boulenger. Tailless Batrachians of Europe. Ray Society, 1897. 



Braus. Anat. Anzeiger, xxvi, 1905. 



Budgett. Quart. Journ. Mier. Sci., xlii, 1899. 



Burrows. Journ. Experim. Zool., x, 1911. 



Cameron. Proc. Roy. Soc. Edinb., xxiv, 1903. 



Cameron. Proc. Roy. Soc. Edinb., xxv, 1904. 



Coggi. Atti Accad.Lincei, Anno 286, Ser. iv, Mem. Class. Sci. fis., vi, 1889, 



