XI TI 



PHYLrM CHORDATA 



];« 



na. rqh) leads by a short pas^ao-o into a rounded olfnrfory sar 

 (Fif(. 796 na, Fig. 79<S) placed just in front of the brain and having 

 its posterior wall raised into ridges covered by the olfactory 

 mucous membrane (Fig. TOS, o//'. ///. m.). From the bottom 

 of the sac is given off a lai-ge pitttit(tiii punrh (Fig. 79(), na', 

 Fig. 798, pt/f. 'p.) which extends downwards and backwards, be- 

 tween the brain and the skull-Hoor, passes through the basici'anial 

 fontanelle, and ends blindly below the anterior end of the 

 notochord. 



The relations between the olfactory sac, the pituitary pouch, and 

 the pituitary body are very remai-kable. Jn the euibr}o, bel'ore 



pn 



Pflf''^lp'>>^^^-1^lp 



olfM,, V-^ inf 



std.m./l2p p/i 



nch 



Fio. 7'.W.— Petromyzon. Diagrams of four stages in tlio development of tlie olfactory and 

 r>ituit:uy sats. ///•. brain; e/(^ mesenteron ; oif. infinulilj\ihnn ; /. /y). lower lip; nc]i. noto- 

 chord ; off. .<. olfactory sac ; pii. iiineal Ijody ; //?//. .s. jiitiiitary sac ; aldia. stomodai^um ; 

 '«. III. upper lip. (Altered fiom Uohni. ) 



the stomodaeum (Fig. 799, A, stdm.) communicates with the mesen- 

 teron, two unpaired ectodermal invaginations appear in front of 

 the mouth. The foremost of these is the rudiment of the olfac- 

 tory sac (o//. s.). The other, which is situated between the olfactory 

 sac and the mouth, is the pituitary sac {pty. s.), which in this case 

 opens just outside the stomoda?um instead of within it as in other 

 Craniata : its inner or blind end extends to the ventral surface of 

 the fore-brain and terminates just below the infundibulum (inf.). 

 As development goes on, the olfactory and pituitary invaginations 

 become sunk in a common pit (B), which, by the growth of the 



