Fig. 311. Head of a Chick from below on the sixth and seventh days of incubation. (From Huxley.)
Ia. cerebral vesicles; a. eye, in which the remains of the choroid slit can still be seen in A; g. nasal pits; k. frontonasal process; l. superior maxillary process; 1. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first visceral cleft.
In A the cavity of the mouth is seen enclosed by the frontonasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over.
In B the external opening of the mouth has become much constricted, but it is still enclosed by the frontonasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below.
The superior maxillary processes have united with the frontonasal process, along nearly the whole length of the latter.
On the sixth day (Born, 394) the sides of this groove unite together in the middle, and convert it into a canal open at both ends—the ventral openings of the canals of the two sides being placed just within the border of the mouth, and forming the posterior nares; while the external openings form the anterior nares. The upper part of the canal, together with the original nasal pit, is alone lined by olfactory epithelium; the remaining epithelium of the nasal cavity being indifferent epiblastic epithelium. Further changes subsequently take place in connection with the posterior nares, but these are described in the section dealing with the mouth.
In Mammalia the general formation of the anterior and posterior nares is the same as in Birds; but, as shewn by Dursy and Kölliker, an outgrowth from the inner side of the canal between the two openings arises at an early period; and becoming separate from the posterior nares and provided with a special opening into the mouth, forms the organ of Jacobson. The general relations of this organ when fully formed are shewn in [fig. 312].
In Lacertilia the formation of the posterior nares differs in some particulars from that in Birds (Born). A groove is formed leading from the primitive nasal pit to the mouth, bordered on its inner side by the swollen edge of the frontonasal process, and on its outer by an outer-nasal process; while the superior maxillary process does not assist in bounding it. On the inner side of the narrowest part of this groove there is formed a large lateral diverticulum, which is lined by a continuation of the Schneiderian epithelium, and forms the rudiment of Jacobson’s organ. The nasal groove continues to grow in length, but soon becomes converted into a canal by the junction of the outer-nasal process with the frontonasal process. This canal is open at both ends: at its dorsal end is placed the original opening of the nasal pit, and its ventral opening is situated within the cavity of the mouth. The latter forms the primitive posterior nares. The superior maxillary process soon grows inwards on the under side of the posterior part of the nasal passage, and assists in forming its under wall. This ingrowth of the superior maxillary process is the rudiment of the hard palate.
Fig. 312. Section through the nasal cavity and Jacobson’s organ. (From Gegenbaur.)
sn. septum nasi; cn. nasal cavity; J. Jacobson’s organ; d. edge of upper jaw.
On the conversion of the nasal groove into a closed passage, the opening of Jacobson’s organ into the groove becomes concealed; and at a later period Jacobson’s organ becomes completely shut off from the nasal cavity, and opens into the mouth at the front end of an elongated groove leading back to the posterior nares.
In Amphibia the posterior nares are formed in a manner very different from that of the Amniota. At an early stage a shallow groove is formed leading from the nasal pit to the mouth; but this groove instead of forming the posterior nares soon vanishes, and by the growth of the front of the head the nasal pits are carried farther away from the mouth.
The actual posterior nares are formed by a perforation in the palate, opening into the blind end of the original nasal pit.