100 THE THIED DAY. [CHAP. 



order to be seen must be looked for on the under surface 

 of the optic vesicle. In this position it is readily recog- 

 nized in the transparent embryo of the third day, Figs. 25 

 and 29. 



Bearing in mind these relations of the gap to the optic 

 stalk, the reader will understand how sections of the optic 

 vesicle at this stage present very different appearances 

 according to the plane in which the sections are taken. 



When the head of the chick is viewed from underneath 

 as a transparent object the eye presents very much the ap- 

 pearance represented in the diagram Fig. 29. 



D. Diagrammatic section taken perpendicular to the plane of the paper, 

 along the line y t y, Fig. 29. The stalk is not seen, the section falling 

 quite out of its region, vh, hollow of optic cup filled with vitreous 

 humour; other letters as in Fig. -27 B. 



E. Section taken parallel to the plane of paper through Fig. 29, so far behind 

 the front surface of the eye as to shave off a small portion of the posterior 

 surface of the lens I, but so far in front as not to be carried at all through 

 the stalk. Letters as before ; /, the choroidal fissure. 



F. Section along the line z, z, perpendicular to the plane of the paper, to shew 

 the choroidal fissure /, and the continuity of the cavity of the optic stalk 

 with that of the primary optic vesicle. Had this section been taken a 

 little to one side of the line 2, z, the wall of the optic cup would have 

 extended up to the lens below as well as above. Letters as before. 



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

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

 corresponding to that of Fig. 30 D. The lens, the cavity 

 and double walls of the secondary vesicle, the remains of the 

 primary cavity, would all be represented (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 



