THE STOAWD/EUM. 



r,37 



In Ascidians this pit is placed on the dorsal surface (fig. 9, o), and 

 becomes the permanent oral cavity of these forms. In the larva of 

 Amphioxiis it is stated to be formed unsymmetrically {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. 410, m) than in any other known 

 form. 



From the primary buccal cavity or stomodgeum there grows out 

 the pituitary pit (Hg. 42(J, pt), the de- 

 velopment of which has already been 

 described (p. 358J. 



The wall separating the stomodseum 

 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 secondary 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 

 opening of the nasal sack, which thence- 

 forward 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 supe- 

 rior maxillary processes (fig. 427, p), 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 [n), 

 containing the opening of the posterior nares, and a ventral cavity, 

 forming tbe permanent mouth. The two divisions thus formed open 

 into a common cavity behind. The horizontal septum, on the de- 

 velopment within it of an osseous plate, constitutes the hard palate. 



An internasal septum (fig. 427, e) may more or less completely 

 divide the dorsal cavity into two canals, continuous respectively 

 with the two nasal cavities. 



In Mammalia a posterior prolongation of the palate, in which an 

 osseous plate is not formed, constitutes the soft palate. 



Fig. 426. Longitudinal sec- 

 tion THROUGH THE BRAIN OF A 

 YOUNG PbISTIURUS EMBRYO. 



cer. unpaired rudiment of the 

 cerebral hemispheres ; p«. pineal 

 gland; In. infundibulum ; pt. 

 ingrowth from mouth to form 

 the pituitary body; inb. mid- 

 brain; cb. cerebellum; ch. noto- 

 chord; ah alimentary tract; laa. 

 artery of mandibular arch. 



