184 Vertebrate Embryology 



day. The lacrymal duct is also at first solid: it ap- 

 pears as a ridge of epidermis, along the line of the 

 lacrymal groove, extending from the eye to the ol- 

 factory pit. This ridge sinks into the mesoblast, and 

 soon splits off from the epiblast for a greater part 

 of its length, but remains attached at its ends to the 



pet 



Fig. 61. — The eye of a bird (columea livia). A, in sagittal 

 section ; B, external view of entire organ. (After Parker and Has- 

 well, from Vogt and Yung.) 



CM, cornea, ch^ choroid, c/./r, ciliary process. ?>, iris. ^,_ lens, opt.nv, 

 optic nerve, ^ct^ pecten. rt^ retina, scl, sclerotic, scl.pl^ sclerotic plates. 



lower eyelid and to the wall of the olfactory pit re- 

 spectively. About the twelfth day it acquires a 

 central lumen, and becomes the tubular duct." ' 



The ear. The ears begin about the middle 

 of the second day, as has been mentioned, 



' Marshall. 



