In all the higher types the nasal pits have originally only a single opening, and the ontogenetic process by which the posterior nasal opening is formed has been studied in the Amniota and Amphibia. Amongst the Amniota we may take the Chick as representing the process in a very simple form. The general history of the process was first made out by Kölliker.
Fig. 309. Section through the brain and olfactory organ of an embryo of Scyllium. (Modified from figures by Marshall and myself.)
c.h. cerebral hemispheres; ol.v. olfactory vesicle; olf. olfactory pit; Sch. Schneiderian folds; I. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain.
Fig. 310. Diagrammatic vertical section through the head of a larva of Petromyzon.
The larva had been hatched three days, and was 4.8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues.
c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum; pn. pineal gland; mb. mid-brain; cb. cerebellum; md. medulla oblongata; au.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart; ch. notochord.
The opening of the nasal pit becomes surrounded by a ridge except on its oral side. The deficiency of this ridge on the side of the mouth gives rise to a kind of shallow groove leading from the nasal pit to the mouth. The ridge enveloping the opening of the nasal pit next becomes prolonged along the sides of this groove, especially on its inner one; and at the same time the superior maxillary process grows forwards so as to bound the lower part of its outer side. The inner and outer ridges, together with the superior maxillary process, enclose a deep groove, connecting the original opening of the nasal pit with the mouth. The process just described is illustrated by [fig. 311] A, and it may be seen that the ridge on the inner side of the groove forms the edge of the frontonasal process (k).
