PHYLUM CHORDATA 



133 



na. ap.) leads by a short passage into a rounded olfactory sac 

 (Fig. 796 na, Fig. 798) placed just in front of the brain and having 

 its posterior wall raised into ridges covered by the olfactory 

 mucous membrane (Fig. 798, olf. m. m). From the bottom 

 of the sac is given off a large pituitary pouch (Fig. 796, na, 

 Fig. 798, pty. p.) which extends downwards and backwards, be- 

 tween the brain and the skull-floor, passes through the basicranial 

 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 remarkable. In the embryo, before 



Tvch 



Fig. 799.— 7etromyzon. Diagrams of four stages in the development of the olfaetory and 

 pituitaiy sacs. hr. brain ; ent. inesenteron ; inf. infundibuluni ; L Ip. lower lip ; nch. noto- 

 chord ; olf. s. olfactory sac ; pn. pineal body ; :piy. s. pituitary sac ; sidm. stomodaium ; 

 u. Ip. upper lip. (Altered fiom Uohm.) 



the stomodseum (Fig. 799, A, stdni.) 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 {olf. 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 stomodseum 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 



