180 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



wall is composed of a single layer of epithelium. The pars 

 ciliaris retinae retains this form. The cells of the sensitive 

 retina elongate, but the process of formation of rods and cones 

 and other structures in the retina has not been fully followed. 

 If Gaskell is right then the matter is simple. He believes that 

 the epithelial cells of the inner layer of the optic cup become 

 transformed into the fibres of Miiller ; the rods, cones, and gang- 

 lionic cells being derived from the cells which grow into the optic 

 vesicle from the epiblast. The ganglionic cells, however, are 

 more probably derivatives of the neuroblasts of the optic vesicle. 

 At any rate an inner layer of ganglionic cells is formed 

 which give off the optic fibres as processes. These fibres 

 converge at the stalk of the vesicle, thus forming the optic 

 disc ; they grow inwards on the stalk, which becomes the optic 

 nerve ; some at least, perhaps all, cross in the floor of the 3rd 

 ventricle forming the chiasma, and pass round, as the optic tracts, 

 to ganglia situated on the mid-brain. There are also efferent 

 fibres in the optic nerve, which end round the ganglion cells of 

 the retina. The inner ganglionic cells of the retina probably 

 correspond to the cells of a posterior root ganglion. According 

 to Gaskell the optic vesicles arose in the ancestry of the verte- 

 brates as diverticula of their alimentary canal ; when the alimentary 

 function of the canal was lost and it became neural, these diverti- 

 cula became the optic vesicles. 



Tie choroidal Fissure. — Occasionally congenital fissures are seen 



hyaloid art. 



/ ^^^\ sS ^^^ mar 9i n °f Pupil 

 optic stalk ^^ 



choroidal fissure 



Fin. 147.— The Optic Stalk and Cut), viewed on the lower and lateral aspect, showing the 

 closure of the Choroidul Fissure. 



in the lower and outer segment of the iris (coloboma iridis). A 

 white line, due to absence of pigment, may be seen in the corre- 



