192 E VOL UTION AND DISEA SE. 



facts connected with the development of the mouth in 

 order to show that we have here to deal with an arrest 

 of development, as well as to show the reason why the 

 cleft in the dog's nose is median, whereas in man and 

 several other mammals, in which the deformity has 

 been recognized, the split is, as a rule, lateral. Fortu- 

 nately the admirable researches of Professor His 

 remove much of the difficulty which would, otherwise 

 have existed, and the account of 

 this embryologist will be closely 

 followed. 



The mouth of the human em- 

 bryo at the fifth week of intra- 

 uterine life is represented by an 

 opening from which five fissures 

 radiate. The upper pair are the 

 orbito-nasal, the two lower form 

 the mouth, whilst the median 

 fissure separates the lower jaws. 

 * As the median process develops 



FIG. 104. Human embryo of r 



the fifth week showing the to form the n OSe, two rounded 



parts out of which the lips, prominences make their appear- 



mouth, and nose are formed. 



ance at each angle. These will 



be referred to as the globular processes (fig. 104) ; these 

 processes furnish the alse of the nose and the inter- 

 maxillae ; later they are joined by the lateral pieces to 

 complete the lip. 



In some mammals, especially rodents, the globular 

 processes fuse with the lateral pieces but do not fuse* 

 with each other and remain permanently separated, thus 

 explaining the occurrence of a persistent median cleft in 



