278 THE CHICK. 



(Fig. 122, OC, OD), and by the end of the third day the original 

 cavity of the optic vesicle is practically obliterated. The 

 whole cup grows rapidly ; its lip remains in contact with the 

 margin of the lens the whole way round, except at one point on 

 the under surface of the cup, below the reference line, OL, in 

 Fig. 122, where a small chink is left between the lens and the lip 

 of the cup. As the optic cup increases in size, this chink becomes 

 lengthened out into a slit, the choroidal fissure (Figs. 113, 

 115, and 125, OH), through which the mesoblast of the head 

 gains admittance into the cavity of the cup. 



From the wall of the optic cup the retina is developed, while 

 the mesoblast which grows into the cavity of the cup, through the 

 choroidal fissure, gives rise to the vitreous body. The choroid 

 and sclerotic coats of the eye are formed from the mesoblast 

 outside the optic cup, and the cornea from mesoblast which 

 grows in between the lens and the surface epiblast. 



The exact mode of formation of the choroidal fissure is 

 difficult to determine. The first step in the doubling up of the 

 optic vesicle to form the optic cup (Fig. 121) is intimately 

 associated with the ingrowth of the surface epiblast to form the 

 lens vesicle, and is perhaps due, in part, to mechanical in-pushing 

 by this latter : the later stages of the doubling up, however, 

 concern the optic cup alone, and must be regarded as due to 

 unequal rates of growth of different parts of the wall of the cup. 

 This unequal rate of growth in different directions probably 

 plays an important, or even predominant, part in the formation 

 of the choroidal fissure. The formation of the choroidal fissure 

 has been recently shown to be closely associated with the 

 growth of the fibres of the optic nerves ; these fibres passing 

 through the choroidal fissure on their way from the retina 

 towards the brain. 



The choroidal fissure only remains open for a short time. 

 About the sixth day its lips come in contact, and very shortly 

 afterwards they fuse together, so as to complete the closure of 

 the optic cup ; by the ninth day all trace of the fissure has dis- 

 appeared. 



The retina is formed directly from the wall of the optic cup. 

 Of the two layers of which the doubled-up wall of the cup 

 consists, the inner (Fig. 122, oc) is from the first much the 

 thicker. It consists, on the third day, of elongated nucleated 



