192 EVOLUTION AND DISEASE. 
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. 
bryo at the fifth week of intra- 1 
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 
Fic. 1o4.—Human embryo of f | a 
the fifth week showing the to form the nose, two rounde 
parts out of which the lips, prominences make their appear- 
mouth, and nose are formed. J 
ance at each angle. These will 
be referred to as the globular processes (fig. 104); these 
processes furnish the ale of the nose and the inter- 
maxilla ; 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 
The mouth of the human em- | 
