ORGANS OF VISION OF THE VERTEBRATA. 



487 



object the eye presents very much the appearance represented in 

 the diagram (fig. 287). 



A section of such an eye taken along the line y, perpendicular 

 to the plane of the paper, would give a figure corresponding 

 to that of fig. 288 D. The lens, 

 the cavity and double walls of the 

 secondary vesicle, the remains of the 

 primary cavity, would all be repre- 

 sented (the superficial epiblast of 

 the head would also be shewn) ; 

 but there would be nothing seen of 

 either the stalk or the fissure. If 

 on the other hand the section were 

 taken in a plane parallel to the 

 plane of the paper, at some distance 

 above the level of the stalk, some 

 such figure would be obtained as 

 that shewn in fig. 288 E. Here the 

 fissure / is obvious, and the com- 

 munication of the cavity vh of the 

 secondary vesicle with the outside 

 of the eye evident ; the section of 

 course would not go through the 

 superficial epiblast. Lastly, a sec- 

 tion, taken perpendicular to the 

 plane of the paper along the line z, 

 i.e. through the fissure itself, would 

 present the appearances 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 external epi- 

 blast 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. 

 Firstly it is extremely doubtful whether the invagination 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 



FIG. 287. DIAGRAMMATIC RE- 

 PRESENTATION OF THE EYE OF 

 THE CHICK OF ABOUT THE THIRD 

 DAY AS SEEN WHEN THE HEAD IS 

 VIEWED FROM UNDERNEATH AS A 

 TRANSPARENT OBJECT. 



I. 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 / its cavity, at a lower level 

 than the cup itself and therefore 

 out of focus; the dotted line in- 

 dicates the continuity of the cavity 

 of the stalk with that of the primary 

 vesicle. 



The line z z, through which the 

 section shewn in fig. 288 F is sup- 

 posed to be taken, passes through 

 the choroidal fissure. 



