260 



COMPARATIVE ANATOMY 



supported by rings of cartilage. This tube is continued backwards 

 from the ventral side of the olfactory organ above the mucous 



HO Hyj> 



B 



Ch 



FIG. 190. A, B, C, MEDIAN LONGITUDINAL SECTION THROUGH THE HEAD OF A 

 LARVA OF Petromyzon plancri IN THREE SUCCESSIVE STAGES, TO SHOW THE 

 MODE OF DEVELOPMENT AND RELATIONS OF THE OLFACTORY AND PITUITARY 

 SACS AND THEIR GRADUAL SHIFTING FROM THE VENTRAL TO THE DoRSAL 

 SIDE, OWING TO THE GROWTH OF THE ORAL FUNNEL. (Mainly after Kupffer 

 and Dohrn.) 



Ch, notochord ; Ohio,*, optic chiasma ; ftp, pineal body ; ////, hind-brain ; /////>, 

 pituitary sac ; Jfi-f, infundibulum ; MB, stomodseum ; M H, mid-brain ; OL, 

 UL, upper and lower margins of oral funnel ; /,'O, olfactory sac ; VET, 

 position of endodermic part of gut shown in C (VOD) opening into the 

 stomodseum ; VH, fore-brain. 



membrane of the mouth : in Petromyzon it forms a blind pouch, 

 but in Myxine opens into the oral cavity as a naso-palatine auct. 



Although the olfactory nerve is paired, the first trace of the 

 olfactory sac in the embryo is seen as an unpaired plate, which soon 



