CHAPTER IX 

 ORGANS OF SPECIAL SENSE 



I. The Eye 



The development of the eye up to the stage of 36 somites has 

 been already described. We shall now consider the subsequent 

 changes in the following order: (1) optic cup, (2) vitreous body, 

 (3) lens, (4) anterior chamber, cornea, iris, etc., (5) choroid and 

 sclerotic, (6) the conjunctival sac and eyelids, (7) the choroid fis- 

 sure and the optic nerve. 



1. The optic cup at the stage of 36 somites is composed of 

 two layers, an inner, thicker layer, known as the retinal layer, 

 and an outer, thinner layer, known as the pigment layer; these 

 are continuous with one another at the pupil and choroid fissure. 

 The inner and outer layers come into contact first in the region 

 of the fundus, and the cavity of the original optic vesicle is gradu- 

 ally obliterated. The choroid fissure is in the ventral face of 

 the optic cup; it is very narrow at this time, and opens distally 

 into the pupil; centrally it ends at the junction of optic stalk 

 and cup, not being continued on the stalk as it is in mammals (Fig. 

 157). 



The walls of the optic cup may be divided into a lenticular 

 zone {pars lenticularis or pars cceca) and a retinal zone; the former 

 includes the zone adjacent to the pupil, not sharply demarcated 

 at first from the remainder or retinal zone, but later bounded dis- 

 tinctly by the ora serrata. The retinal zone alone becomes the 

 sensitive portion of the eye; the lenticular zone develops into the 

 epithelium of the iris and ciliary processes. 



In the lenticular zone the inner and outer layers become actu- 

 ally fused, but in the retinal zone they may always be separated; 

 indeed, in most preparations they are separated by an actual 

 space produced by unequal shrinkage. 



The differentiation of the lenticular from the retinal zone 

 begins about the seventh day, when a marked difference in thick- 



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