In the Cephalochorda (Amphioxus) there is a shallow ciliated pit, discovered by Kölliker, which is situated on the left side of the head, and is closely connected with a special process of the front end of the brain. It is most probably the homologue of the olfactory pits of the true Vertebrata.
In the true Vertebrata the olfactory organ has usually the form of a pair of pits, though in the Cyclostomata the organ is unpaired.
In all the Vertebrata with two olfactory pits these organs are formed from a pair of thickened patches of the epiblast, on the under side of the fore-brain, immediately in front of the mouth ([fig. 307], ol). Each thickened patch of epiblast soon becomes involuted as a pit ([fig. 308], N), the lining cells of which become the olfactory or Schneiderian epithelium. The surface of this epithelium is usually much increased by various foldings, which in the Elasmobranchii arise very early, and are bilaterally symmetrical, diverging on each side like the barbs of a feather from the median line. They subsequently become very pronounced ([fig. 309]), serving greatly to increase the surface of the olfactory epithelium. At a very early stage the olfactory nerve attaches itself to the olfactory epithelium.
In Petromyzon the olfactory organ arises as an unpaired thickening of the epiblast, which in the just hatched larva forms a shallow pit, on the ventral side of the head, immediately in front of the mouth. This pit rapidly deepens, and soon extends itself backwards nearly as far as the infundibulum ([fig. 310], ol). By the development of the upper lip the opening of the olfactory pit is gradually carried to the dorsal surface of the head, and becomes at the same time narrowed and ciliated ([fig. 47], ol). The whole organ forms an elongated sack, and in later stages becomes nearly divided by a median fold into two halves.
It is probable that the unpaired condition of the olfactory organ in the Lamprey has arisen from the fusion of two pits into one; there is however no evidence of this in the early development; but the division of the sack into two halves by a median fold may be regarded as an indication of such a paired character in the later stages.
In Myxine the olfactory organ communicates with the mouth through the palate, but the meaning of this communication, which does not appear to be of the same nature as the communication between the olfactory pits and the mouth by the posterior nares in the higher types, is not known.
Fig. 308. Side view of the head of an embryo Chick of the third day as an opaque object. (Chromic acid preparation.)
C.H. cerebral hemispheres; F.B. vesicle of third ventricle; M.B. mid-brain; Cb. cerebellum; H.B. medulla oblongata; N. nasal pit; ot. auditory vesicle in the stage of a pit with the opening not yet closed up; op. optic vesicle, with l. lens and ch.f. choroidal fissure.
1 F. The first visceral fold; above it is seen the superior maxillary process.
2, 3, 4 F. Second, third and fourth visceral folds, with the visceral clefts between them.
The opening of the olfactory pit does not retain its embryonic characters. In Elasmobranchii and Chimæra it becomes enclosed by a wall of integument, often deficient on the side of the mouth, so that there is formed a groove leading from the nasal pit towards the angle of the mouth. This groove is usually constricted in the middle, and the original single opening of the nasal sack thus becomes nearly divided into two. In Teleostei and Ganoids the division of the nasal opening into two parts becomes complete, but the ventral opening is generally carried off some distance from the mouth, and placed, by the growth of the snout, on the upper surface of the head ([figs. 54] and [68]). In all these instances it is probable that the dorsal opening of the external nares, and the ventral opening with the posterior nares of higher types. Thus the posterior nares would in fact seem to be represented in all Fishes by a ventral part of the opening of the original nasal pit which either adjoins the border of the mouth (many Elasmobranchii) or is quite separate from the mouth (Teleostei and Ganoidei). In the Dipnoi, Amphibia and all the higher types the oral region becomes extended so as to enclose the posterior nares, and then each nasal pit acquires two openings; viz. one outside the mouth, the external nares, and one within the mouth, the internal or posterior nares. In the Dipnoi the two nasal openings are very similar to those in Ganoidei and Teleostei, but both are placed on the under surface of the head, the inner one being within the mouth, and the external one is so close to the outer border of the upper lip that it also has been considered by some anatomists to lie within the mouth.