16 



CONARIO-HYPOPHYSIAL TRACT. 



of the myelencephalon, as seen in the embiyos of Cyclosto- 

 mous fishes*. In fig. 7 these enlargements are represented 

 by the figures 4, 5, 6, the latter now pushing beyond the 

 notochord, 2. Toward the middle one of these extends the 

 tubular production ( 8') from the digestive sac (ii). In the 

 line opposite to the produc- 

 tion ( 8') is an infolding of the 

 ectoblast (7), which Scott in- 

 dicates as that of the nasal 

 cavity and hypophysis (" ge- 

 meinsame Einbuchtung flir 

 Nasengrube und Hypophysis," 

 loc. cit. p. 171, Taf. ix.fig. 31, 

 N. H. E.). Beneath this has 



commenced the wider infold- Enlarged scale of a longitudinal 

 ing of the ectoblast (fig. 7,9'), vertical midsection of an embryo 



which, extending backward, PlaneH at the 



11 , , T eighteenth day. 



and subsequently expandmg ^ 



and developing the branchial sacs, ultimately effects a com- 

 munication with the alimentary cavity (11), and establishes 

 the permanent oral entry thereto. In the more highly 

 organized cartilaginous fishes (Elasmobranchs) the haemal 

 permanent mouth, or " tritostome," is also due to involution 

 of the epiblast, forming a sac, beneath the base of the brain, 

 the closed end of the sac coming into contact with the fore 

 end of the alimentary cavity, developing upward the in- 

 fundibulum. Balfour sees the rudiment of the hypophysis 



* See Owsjannikow, " Die EutwicHungsgeschichte der Petromyzon 

 Jluviatilis," Bulletin de I'Acad. Imp. St. Petershourg, torn, xiv. 1870, 

 p. 325 ; Calberla, Morpholog. Jahrbuch, Bd. iii. p. 226 ; Scott (W. B.), 

 Morpholog. Jahrbuch, Bd. vii., erstes Heft, p. 131, " Beitriige zur Ent- 

 wicklungsgeschichte der Petromyzonten," from which treatise the subject 

 of fig. 7 is taken. 



