250 THE FIFTH DAY. [CHAP. 



FIG. 82. 



HEAD OF A CHICK OF THE SEVENTH DAY FROM BELOW. (From 

 Huxley.) 



la. cerebral vesicles, a. eye. g. nasal pits. L fronto-nasal 

 process. 1. superior maxillary process. 1. first visceral 

 arch. 2. second visceral arch. x. first visceral cleft. 



The external opening of the mouth has become much con- 

 stricted, 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 the whole length of the latter, with the 

 exception of a small space in front, where a narrow angular 

 opening is left between the two. 



processes unite together to form the upper boundary of 

 the mouth, each superior maxillary process sends in- 

 wards a lateral bud. These buds become flattened and 

 form horizontal plates which stretch more and more 

 inward towards the middle line. There they finally 

 meet, and by their union, which is effected first in front 

 and thence extends backwards, constitute a horizontal 



