VI.] THE EYE. 141 



the eyeball is definitely formed. The internal portions 

 of this investment, nearest to the retina, become the 

 choroid {i.e. the chorio-capillaris, and the lamina 

 fusca, the pigment epithelium, as we have seen, being 

 derived from the epiblastic optic cup), and pigment is 

 subsequently deposited in it. The remaining external 

 portion of the investment forms the sclerotic. 



The complete differentiation of these two coats 

 of the eye does not however take place till a late 

 period. 



In front of the optic cup the mesoblastic invest- 

 ment grows forwards, between the lens and the super- 

 ficial epiblast, and so gives rise to the substance of 

 the cornea ; the epiblast supplying only the anterior 

 epithelium. 



We may now proceed to give some further details 

 with reference to the histological differentiation of the 

 parts, whose general development has been dealt with 

 in the preceding pages. 



The histological condition of the eye in its earliest 

 stages is very simple. Both the epiblast forming the 

 walls of the optic vesicle, and the superficial layer 

 which is thickened to become the lens, are composed of 

 simple columnar cells. The surrounding mesoblast is 

 made up of cells whose protoplasm is more or less 

 branched and irregular. These simple elements are 

 gradually modified into the complicated tissues of the 

 adult eye, the changes undergone being most marked 

 in the cases of the retina, the optic nerve, and the 

 lens with its appendages. 



The optic vesicle. We left the original cavity of 

 the primary optic vesicle as a nearly obliterated space 



