THE STOMOD/EUM. 



775 



(vide p. 5), but further observations on its development are 

 required. 



In the true Vertebrata it is always formed on the ventral 

 surface of the head, immediately behind the level of the fore- 

 brain (fig. 426), and is deeper in Petromyzon (fig. 416, ;) than 

 in any other known form. 



From the primary buccal cavity or stomodaeum there grows 

 out the pituitary pit (fig. 426, pt\ the 

 development of which has already 

 been described (p. 435). 



The wall separating the stomo- 

 daeum from the mesenteron always 

 becomes perforated, usually at an 

 early stage of development, and 

 though in Petromyzon the boundary 

 between the two cavities remains 

 indicated by the velum, yet in the 

 higher Vertebrata all trace of this 

 boundary is lost, and the original 

 limits of the primitive buccal cavity 

 become obliterated ; while a secon- 

 dary buccal cavity, partly lined by 

 hypoblast and partly by epiblast, 

 becomes established. 



This cavity, apart from the organs which belong to it, 

 presents important variations in structure. In most Pisces it 

 retains a fairly simple character, but in the Dipnoi its outer 

 boundary becomes extended so as to enclose the ventral open- 

 ing of the nasal sack, which thenceforward constitutes the 

 posterior nares. 



In Amphibia and Amniota the posterior nares also open well 

 within the boundary of the buccal cavity. 



In the Amniota further important changes take place. 



In the first place a plate grows inwards from each of the 

 superior maxillary processes (fig. 427, /), and the two plates, 

 meeting in the middle line, form a horizontal septum dividing 

 the front part of the primitive buccal cavity into a dorsal 

 respiratory section (), containing the opening of the posterior 

 nares, and a ventral cavity, forming the permanent mouth. The 



FIG. 426. LONGITUDINAL 

 SECTION THROUGH THE BRAIN OF 

 A YOUNG PRISTIURUS EMBRYO. 



r.unpaired rudimentofthecere- 

 bral hemispheres \pn. pineal gland ; 

 /w.infundibulum ; //.ingrowth from 

 mouth to form the pituitary body ; 

 mb. mid-brain ; cb. cerebellum ; ch. 

 notochord; al. alimentary tract; 

 Zaa. artery of mandibular arch. 



