The Eye 299 



bryonic life, varying apparently in species from 10-20 days after 

 fertilization. The optic vesicles appear first as tubular out- 

 growths from the forebrain, but thedistal end of eachsoon becomes 

 enlarged, while the connecting stalk remains a narrow tube. Later, 

 the enlarged distal end becomes invaginated within itself to con- 

 stitute the optic cup, which is later to form the retina and within 

 which the vitreous body develops. The lens develops somewhat 

 later, first appearing as a pit in the epiblast upon the side of the 

 head. This pit gradually siuks deeper and its mouth narrows, 

 until it finally closes to constitute the vesicle of the lens. 



The vesicle sinks into the optic cup and the margins of the latter 

 come in close contact with it except at one point on the ventral 

 surface, where there is a distinct groove, the choroidal fissure. 

 Within the vesicle the lens becomes elaborated from the enclo-sed 

 epiblastic cells. 



The vitreous body is derived from an ingrowth of mesoblast, 

 which enters the optic cup through the choroidal fissure. 



As the lens becomes invaginated and separated from the 

 external epiblast, a la5'er of mesoblast extends across between 

 the vesicle and the external epiblast, constituting the cornea, the 

 tissues of which become transparent, The choroid and sclerotic 

 coats develop from the mesoblastic tissues surrounding the optic 

 cup ; the iris is formed by a forward growth of the margins of the 

 optic cup ; and the anterior chamber appears somewhat later as an 

 excavation between the cornea and lens. The eyelids are the last 

 structures to form in connection with the eye and consist of 

 folds of the skin from above and below. These extend over the 

 eyeball to finally meet and become fused together, without adher- 

 ing to the surface of the cornea, so that they create a closed sac, 

 the lacrymal sac, between their internal surfaces and the cornea. 

 In the rabbit and carnivora, the eyelids remain closed for a short 

 time after birth, while, in ruminants and solipeds, the eyelids 

 open somewhat prior to birth. 



The membrana nictatans, or third eyelid, is formed from a fold 

 of skin in a similar manner to the two ordinary eyelids and ex- 

 ists in all domestic mammals. 



The lacrymal duct is formed in the groove existing between 

 the external nasal process and the maxillary arch. 



In the domestic animals, we occasionally meet with aberrations 

 in the development of the eye. Sometimes the eyelids have be- 



