Fig. 426. Longitudinal section through the brain of a young Pristiurus embryo.
cer. unpaired rudiment of the cerebral hemispheres; pn. pineal gland; In. infundibulum; pt. ingrowth from mouth to form the pituitary body; mb. mid-brain; cb. cerebellum; ch. notochord; al. alimentary tract; Iaa. artery of mandibular arch.
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], m) than in any other known form.
From the primary buccal cavity or stomodæum there grows out the pituitary pit ([fig. 426], pt), the development of which has already been described (p. [435]).
The wall separating the stomodæum 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 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], 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 the permanent mouth. The two divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate.