552 ME. H. M. KXLE ON NASAL 



moutL and nose appear; and i£ this allegation should hold good 

 also for Myxine, a special importance must be ascribed to such an 

 early communication of the hypophysis with the gut, and it would 

 furoish an argument in favour of Dean's view that the Myxinidse 

 and the Petromyzontidse stand to each other in somewhat the 

 same relation as the recent Selachians to the recent Ganoids. 

 But, however widely they may be separated, the hypophysis would 

 appear to present a condition analogous in the one group to 

 that of the closed nasal-sac state aforementioned, and in the 

 other to that of the open nasal canals in Cynoglossus. 



The assertion of Dean that the hypophysis in Bdellostonia 

 opens into the gut before the external opening of the nasal 

 organ is formed, if correct, further leads to the conclusion that 

 the so-called " internal nares " of the Myxinidse, although a 

 primary formation, is only secondarily connected with respiration. 

 The reverse is the case in Cynoglossus, and thus there arises 

 a good example of convergency in evolution, since a similar 

 structure possessing a similar function, but having a totally 

 different origin, would appear to have arisen in two separate 

 groups of the animal kingdom. 



On the other hand, there is so much diversity of opinion with 

 regard to the development of the internal nares in the higher 

 Vertebrates, that reference to them is somewhat difficult. 

 According to Balfour's theory *, the nares arise from a single 

 depression lying anterior to, and one on each side of, the mouth. 

 This depression, as the embryo develops, takes the form of a 

 longitudinal slit, and a little later passes through a stage similar 

 to that of the adult Selachian. By the fusion of the adjacent 

 tissues over the depression, this slit becomes tranformed into a 

 canal with a,n opening at either end — these openings represent 

 the external and internal nares. The later development differs in 

 the different groups. In the Amniota, Balfour believed that the 

 maxillary region of the face so develops in relation to the canal 

 that the two openings become widely separated, the outer passing 

 upwards and forming the external nares, the inner passing 

 inwards and forming the internal nares. In the Amphibia it was 

 believed that the origin of the internal nares is distinct from that 

 above described, being " secondary," through perforation of the 

 roof of the mouth after the latter is developed ; and the develop- 

 ment of the nares in the Dipnoi was considered to be similar to 



* Balfour, F. M. : ' Comparative Embryology,' vol. ii, pp. 533-538. 



