166 ON THE MORPHOLOGICAL CONSTITUTION OF 



metasomatomic clefts, we may expect to find traces of these 

 clefts in the walls of the adult nasal fossae. 



The first or anterior pair of metasomatomic clefts of the 

 embryo head, that is the clefts between the rhinal and inter- 

 maxillary lobe of the " median frontal process," are retained 

 in the adult as the external nares. These openings in the 

 non-proboscidian mammal, are situated therefore between the 

 ali-nasal cartilages and the intermaxillary bones. In the pro- 

 boscidian mammals, they are probably situated between the 

 ultimate and penultimate, or, at least, between two of the 

 distal somatomes of the proboscis. 



The second pair of metasomatomic clefts, situated between 

 the external angles of the median frontal process and the 

 lateral frontal processes, may disappear entirely in the course 

 of development ; but they occasionally remain under the form 

 of Stenson's ducts, which pass obliquely through the so-called 

 " incisive spaces," or " foramina," from the mouth to the nasal 

 fossae, between the intermaxillaries and maxillaries. The 

 mucous walls of the canals of Stenson are supported by 

 cartilaginous tubular folds, which are continuous superiorly 

 with cartilaginous lamina, which, passing off laterally from 

 the lower margin of the nasal septum and vomer, cover more 

 or less of the floor of the nasal fossae, upper part of the incisive 

 fissures, and spaces between the intermaxillaries and maxilla- 

 ries. The " organs " or " sacs of Jacobson/' supplied by the 

 olfactory and fifth nerves, lined by glandular integument, 

 sheathed by a continuation of the cartilaginous laminae 

 already alluded to, and opening into the canals of Stenson, 

 when these are present, are, whatever their function may be, 

 morphologically connected with the second pair of meta- 

 somatomic clefts. 



The next pair of metasomatomic clefts, situated between 

 the lateral frontal processes and the so-called " superior 

 maxillary " deflection of the " first visceral lamina/' con- 



