VII.] THE NASAL LABYRINTH. 181 



however, it is separated by a shallow depression. This de- 

 pression, which runs nearly horizontally outwards towards the 

 eyeball, is, according to Coste and Kolliker, subsequently con- 

 verted into the lachrymal duct. 



On the fifth day, the inner nasal processes or lower and 

 outer corners of the fronto-nasal process arching over, unite 

 on each side with the superior maxillary processes. (Com- 

 pare Fig. 57, which, however, is a view of the head of a 

 chick , of the sixth day.) In this way each nasal groove is 

 converted into a canal, which leads from the nasal pit above, 

 into the cavity of the mouth below, and places the two in 

 direct communication. This canal, whose lining consists of 

 epiblast, is the rudiment of the nasal labyrinth. 



FIG. 57. 



HEAD OF A CHICK AT THE SIXTH DAY FROM BELOW. (Copied from Huxley's 

 Elements of Comparative Anatomy.) 



la. cerebral vesicles, a. eye, in which the remains of the choroid slit can still be 

 seen. g. nasal pits. k. fronto-nasal process. I. superior maxillary process. 

 I. inferior maxillary process or first visceral arch. 2. second visceral arch. 

 x. first visceral cleft between the first and second visceral arches. 

 The cavity of the mouth is seen enclosed by the fronto-nasal process, the 

 superior maxillary processes and the first pair of visceral arches. At the back 

 of it is seen the opening leading into the throat. The nasal grooves leading 

 from the nasal pits to the mouth are already closed over and converted into 

 canals. . 



By the seventh day (Fig. 58), not only is the union of 

 the superior maxillary and fronto-nasal processes completed, 

 and the upper boundary of the mouth thus definitely con- 



