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 



set 



cl.p 



pet 



FIG. 61. THE EYE OF A BIRD (COLUMBA LIVIA). A, in sagittal 

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

 well, from Vogt and Yung.) 



en, cornea, ch, choroid. cl.pr, ciliary process, ir, iris. /, lens, opt.nv, 

 optic nerve, pet, 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, 



1 Marshall. 



