102 v< THE T&IRD DAY. [CHAP. 



may be^alraady ^V^lt^tli^n^y^r'Qf-me^oblaRt, interposed between it and the 

 optic vesicle. This layer is carried inward during the involution, and from 

 it both the vitreous humour and the capsule of the lens take their origin. 

 Jn birds it is very difficult to be sure of the existence of this layer, though 

 Lieberkiihn says that in mammals it is conspicuous ; and even if its existence 

 be admitted, it still remains doubtful whether it gives rise to the whole vi- 

 treous humour, or to the capsule of the lens only; though the latter view is most 

 probable. 



During the changes in the optic vesicle just described, the 

 surrounding mesoblast takes on the characters of a distinct 

 investment, whereby the outline of 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 pig- 

 ment 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. Along the 

 line of the choroidal fissure the pigment is wanting. Con- 

 sequently in embryos of an age when the pigment has be- 

 come generally deposited in the choroid, a colourless streak 

 marking out the position of the choroidal fissure is very con- 

 spicuous. 



In front of the optic cup the mesoblastic investment 

 grows forwards, between the lens and the superficial epi- 

 blast, and so gives rise to the substance of the cornea; the 

 epi blast supplying only the anterior epithelium. 



At first the whole space between the lens and the super- 

 ficial epiblast is occupied by undifferentiated mesoblast; but 

 on the sixth day a layer of epithelium makes its ap- 

 pearance in midst of the mass, and thus divides it into an 

 anterior and a posterior portion. The anterior portion, in- 

 creasing in solidity, becomes the cornea, and remains con- 

 tinuous with the sclerotic; the epithelium in question per- 

 sisting as the posterior epithelium of the membrane of 

 Descemet. The posterior portion is reduced to a mere 

 membrane forming, according to Lieberkiihn, the front 

 limb of the capsule (and the suspensory ligament) of the 

 lens, the space between it and the cornea becoming filled 

 with aqueous humour. 



We left the original cavity of the primary optic vesicle as 



