490 EMBRYOLOGY. 



serrata, becomes very much reduced in thickness, and extends over 

 the front surface of the lens, growing into the anterior chamber of 

 the eye until the originally wide opening of the cup is reduced to the 

 size of the pupil. 



12. The anterior attenuated portion of the cup is, in turn, divided 

 into two zones, of which the posterior becomes folded at the periphery 

 of the equator of the lens to form the ciliary processes, whereas in 

 front it remains smooth ; so that in the whole cup three parts 

 may now be distinguished, as retina, pars ciliaris, and pars iridis 

 retinae. 



13. Corresponding to the three portions of the epithelial optic cup, 

 the adjoining connective -tissue envelope takes on somewhat different 

 conditions as the choroid proper, and as the connective-tissue frame- 

 work of the ciliary body and that of the iris. 



14. The skin surrounding the cornea becomes infolded to form the 

 upper and lower eyelids and the nictitating membrane, of which the 

 last is rudimentary in Man, persisting only as the plica semilunaris. 



15. The epithelial layers of the edges of the two eyelids grow 

 together in the last months of development, but become separated 

 again before birth. 



16. The lachrymal groove in Mammals passes from the inner 

 angle of the eye, between the maxillary and outer nasal processes, 

 to the nasal chamber. 



17. The lachrymal duct for carrying away the lachrymal fluid is 

 formed by the downgrowth and constricting off of an epithelial ridge 

 from the bottom of the lachrymal groove, the ridge becoming 

 hollow. 



18. The two lachrymal tubules are developed by the division of the 

 epithelial ridge at the angle of the eye. 



B. The Development of the Organ of Hearing. 



In the case of the ear numerous parts of quite different origin 

 unite, in much the same manner as in the case of the eye, to form a 

 single very complicated apparatus ; of these, too, it is the portion 

 to which the auditory nerve is distributed the membranous labyrinth 

 with its auditory epithelium that is by far the most important, out- 

 stripping as it does all the remaining parts in its development : it 

 must consequently be considered first. 



