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XIl] THE EYE. 387 



the crura cerebri soon interpose themselves. At a 

 sHghtly later period a thin superficial layer of white 

 matter, homologous with that of the remainder of the 



1 brain, becomes established. 

 The inner layer, together with the fibres from the 

 crura cerebri, gives rise to the major part of the white 

 matter of the hemispheres and to the epithelium lining 

 the lateral ventricles. 



The outer layer of rounded cells becomes divided 

 into (1) a superficial part with comparatively few cells, 

 which, together with its coating of white matter, forms 

 the outer part of the grey matter, and (2) a deeper 

 layer with numerous cells, which forms the main mass 

 of the grey matter of the cortex. 



The eyes. The development of the Mammalian eye 

 is essentially similar to that of the chick (ch. vi.) There 

 are however two features in its development which de- 

 serve mention. These are (1) the immense foetal develop- 

 ment of the blood-vessels of the vitreous humour and 

 the presence in the embryo of a vascular membrane sur- 

 rounding the lens, known as the memhrana capsulo- 

 'pupillaris, (2) the absence of any structure comparable 

 to the pecten, and the presence of the arteria centralis 

 retinae. 



In the invagination of the lens (rabbit) a thin 

 I layer of mesoblast is carried before it, and is thus 

 transported into the cavity of the vitreous humour. 

 In the folding in of the optic vesicle which accom- 

 panies the formation of the lens the optic nerve is 

 included, and on the development of the cavity of the 

 vitreous humour an artery, running in the fold of 

 the optic nerve, passes through the choroid slit into the 



252 



