ORGANS OF VISION OF THE VERTEBRATA. 507 



In Lacertilia the lacrymal duct arises very much in the same manner as 

 in Amphibia, though its subsequent growth is somewhat different. It 

 appears as an internal ridge of the epithelium, at the junction of the superior 

 maxillary process and the fold which gives rise to the lower eyelid. A solid 

 process of this ridge makes its way through the mesoblast on the upper 

 border of the maxillary process till it meets the wall of the nasal cavity, with 

 the epithelium of which it becomes continuous. At a subsequent stage 

 a second solid growth from the upper part of the epithelial ridge makes its 

 way through the lower eyelid, and unites with the inner epithelium of the 

 eyelid ; and at a still later date a third growth from the lower part of the 

 structure forms a second junction with the epithelium of the eyelid. The 

 two latter outgrowths form the two upper branches of the duct. The 

 ridge now loses its connection with the external skin, and, becoming 

 hollow, forms the lacrymal duct. It opens at two points on the inner 

 surface of the eyelid, and terminates at its opposite extremity by opening 

 into the nasal cavity. It is remarkable, as pointed out by Born, that the 

 original epithelial ridge gives rise directly to a comparatively small part of 

 the whole duct. 



In the Fowl the lacrymal duct is formed as a solid ridge of the epidermis, 

 extending along the line of the so-called lacrymal groove from the eye to the 

 nasal pit (fig. 120). At the end of the sixth day it begins to be separated 

 from the epidermis, remaining however united with it on the inner side of 

 the lower eyelid. After its separation from the epidermis it forms a solid 

 cord, the lower end of which unites with the wall of the nasal cavity. The 

 cord so formed gives rise to the whole of the duct proper and to the lower 

 branch of the collecting tube. The upper branch of the collecting tube is 

 formed as an outgrowth from this cord. A lumen begins to be formed on 

 the twelfth day of incubation, and first appears at the nasal end. It arises 

 by the formation of a space between the cells of the cord, and not by 

 an absorption of the central cells. 



In Mammalia Kolliker states that he has been unable to observe 

 anything similar to that described by Born in the Sauropsida and Amphibia, 

 and holds to the old view, originally put forward by Coste, that the duct is 

 formed by the closure of a groove leading from the eye to the nose between 

 the outer nasal process and the superior maxillary process. The upper 

 extremity of the duct dilates to form a sack, from which two branches pass 

 off to open on the lacrymal papillae. In view of Born's discoveries Kolliker's 

 statements must be received with some caution. 



The Eye of tJte Tunicata. 



The unpaired eye of the larva of simple Ascidians is situated 

 somewhat to the right side of the posterior part of the dorsal 

 wall of the anterior cephalic vesicle (fig. 296, O\ It consists of 

 a refractive portion, turned towards the cavity of the vesicle of 



