ORGANS OF VISION OF THE VERTEBRATA. 417 



In Teleo.-tei anil Ganoidei eyelids are either not present or at most 

 very rudimentary. In Elasmobranchii they are better developed, and the 

 nictitating membrane is frequently present. The latter is also usually 

 found in Amphibia. In the Sauropsida all three eyelids are usually 

 present, but in Mammalia the nictitating membrane is rudimentary. 



In many Mammalia the two eyelids meet together during a period of 

 embryonic life, and unite in front of the eye. A similar arrangement 

 is permanent through life in Opbidia and some Lacertilia ; and there is a 

 chamber formed between the coalesced eyelids and the surface of the 

 cornea, into which the lacrymal ducts open. 



Laciymal glands. Lacrymal glands are found in the Sauropsida 

 and Mammalia. They arise (Remak, Kolliker) as solid ingrowths of 

 the conjunctival epithelium. They appear in the chick on the eighth day. 



Lacrymal duct. T^ 16 lacrymal duct first appears in Amphibia, and 

 is present in all the higher Vertebrates. Its mode of development in the 

 Amphibia, Lacertilia and Aves has recently been very thoroughly worked 

 out by Born. (Nos. 380 and 381). 



In Amphibia he finds that the lacrymal duct arises as a solid ridge of 

 the mucous layer of the epidermis, continued from the external opening 

 of the nasal cavity backwards towards the eye. It usually appears at 

 about the time when the nasal capsule is beginning to be chondrified. As 

 this ridge is gradually prolonged backwards towards the eye its anterior 

 end becomes separated from the epidermis, and grows inwards in the 

 mesoblast to become continuous with the posterior part of the nasal sack. 

 The posterior end which joins the eye becomes divided into the two col- 

 lecting branches of the adult. Finally the whole structure becomes sepa- 

 rated from the skin except at the external opening, and develops a lumen. 



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 riclge 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 riclge 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 



B. E. II. 27 



