xm 



PHYLUM CHORDATA 



129 



which extends downwards and backwards, between the brain and 

 the skull-floor, passes through the basicranial fontanelle, and ends 

 blindly below the anterior end v of the notochord. 



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

 the pituitary body are very remarkable. In the embryo, before 

 the stomodaeum (Fig. 809, A, stdm.) communicates with the mesen- 



nrh 



rich 



l.lf 



Fio. 809. Petromyzon. Diagrams of four stages in the development of the olfactory and 

 pituitary sacs. br. brain ; ent. mesenteron ; inf. infundibulum ; I. Ip. lower lip ; nch. noto- 

 chord ; olf. s. olfactory sac ; pn. pineal body ; ply. s. pituitary sac ; stdm. stomodseum 

 u. Ip. upper lip. (Altered from Dohrn.) 



teron, two unpaired ectodermal invaginations appear in front of 

 the mouth. The foremost of these is the rudiment of the olfactory 

 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 stomoda3um 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 

 immense upper lip (up. L), is gradually shifted to the top of 

 the head (C, D), the process being accompanied by elongation of the 

 pituitary sac, into which the olfactory sac opens posteriorly. Where 

 the pituitary sac comes in contact with the infundibulum it gives 

 off numerous small follicles which become separated off and give 

 rise to the pituitary body (Fig. 808, pty. b.). Thus the entire nasal 



VOL. II I 



