442 



EXTERNAL AND INTERNAL NARES. 



ings arises at an early period ; and becoming separate from the pos- 

 terior 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 fronto-nasal process, and on its outer by an outei'- 

 nasal process; while the superior maxillary process does not assist in 



Fig. 311. Head of a Chick from below on the sixth and seventh days 

 OF incubation. (From Huxley.) 



J", cerebral vesicles ; a. eye, in which the remains of the choroid slit can still be 

 seen in A ; g. nasal pits ; k. fronto-nasal process ; I. 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 fronto-nasal 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 fronto-nasal 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 fronto nasal process, along 

 nearly the whole length of the latter. 



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 con- 

 tinuation 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 fronto-nasal 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, 



