ORGANS OF VISION OF THE VERTEBRATA. 



401 



fissure f is obvious, and the communication of the cavity vh of the 

 secondary vesicle with the outside of the eve evident; the section 



*/ / 



of course would not go through the 

 superficial epiblast. Lastly, a section, 

 taken perpendicular to the plane of the 

 paper along the line 2, i.e. through the 

 fissure itself, would present the appear- 

 ances of fig. 288 F, where the wall of 

 the vesicle is entirely wanting in the 

 region of the fissure marked by the 

 position of the letter f. The ex- 

 ternal epiblast has been omitted in 

 this figure. 



With reference to the above description, 

 taken with very slight alterations from the 

 Elements of Embryology, Pt. I., two points 

 require to be noticed. Fii'stly it is ex- 

 tremely doubtful whether the imagination 

 of the secondary optic vesicle is to be 

 viewed as an actual mechanical result of 

 the ingrowth of the lens. Secondly it 

 seems probable that the choroid fissure is 

 not simply due to an inequality in the 

 growth of the walls of the secondary optic 

 cup, but is partly due to a doubling up of 

 the primary vesicle from the side along the 

 line of the fissure, at the same time that 

 the lens is being thrust in in front. In 

 Mammalia, the doubling up involves the 

 optic stalk, which becomes flattened (where- 

 by its original cavity is obliterated) and 

 then folded in on itself, so as to embrace 

 a new central cavity continuous with the 

 cavity of the vitreous humour. And in 

 other forms a partial phenomenon of the 

 same kind is usually observable, as is more 

 particularly described in the sequel. 



Before describing the development through the choroidal fissure. 

 of the cornea, aqueous humour, etc. we 



may consider the further growth of the parts, whose first develop- 

 ment has just been described, commencing with the optic cup. 



During the above changes the mesoblast surrounding the optic 

 cup assumes the character of a distinct investment, whereby the out- 

 line of the eye-ball is definitely formed. The internal portions of this 

 investment, nearest to the retina, become the choroid (i.e. the chono- 

 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. 



FIG. 287. DIAGRAMMATIC RE- 

 PRESENTATION OF THE EYE OF THE 

 CHICK OF ABOUT THE THJBD DAY AS 

 SEEN WHEN THE HEAD IS VIEWED 

 FROM UNDERNEATH AS A TRANS- 

 PARENT OBJECT. 



L the lens ; /'. the cavity of the 

 lens, lying in the hollow of the 

 optic cup; r. the anterior, u. the 

 posterior wall of the optic cup ; c. 

 the cavity of the primary optic 

 vesicle, now nearly obliterated. By 

 inadvertence u has been drawn in 

 some places thicker than r, it 

 should have been thinner through- 

 out, s. the stalk of the optic cup 

 with 

 than 



out of focus; the dotted line in- 

 dicates the continuity of the cavity 

 of the stalk with that of the pri- 

 mary vesicle. 



The line z x, through which 

 the section shewn in fig. 288 F is 

 supposed to be taken, passes 



B. E. n. 



