14 THE EMBRYOLOGY OF THE EYE. 



however, it has been questioned whether there 

 is not a continuous thin layer of mesoblastic 

 tissue separating them. 



2. Secondary optic vesicle. — The first step 

 toward the development of the secondary 

 optic vesicle is the thickening of the external 

 epiblast to form the lens. This thickening 

 takes place at the point where the primary 

 optic vesicle is in contact with the external 

 epiblast. As this thickening increases, the 

 adjacent wall of the primary vesicle is pushed 

 in. This is shown in Fig. 2, in which the 

 section is excentric so that the beginning lens- 

 sac is cut through its periphery and hence 

 appears simply as a thickening and not as a 

 cupping, which is shown in Fig. 3. In Fig. 2 

 the section also passes outside the pedicle of 

 the vesicle, and the latter appears therefore as 

 a closed sac. 



With its further growth the outer or distal 

 wall of the primary vesicle becomes still 

 further involuted (Fig. 3), although for a time 

 the two walls are not in contact in their entire 



