lO THE EMBRYOLOGY OE THE EYE. 



The lids are formed by folds of eplblast 

 thrown out above and below, into which meso- 

 blastic tissue pushes. These folds as they 

 develop cover the cornea and finally meet. 

 The epithelium of either lid margin proliferates 

 and joins with that of its fellow, thus connect- 

 ing the lids securely together and forming a 

 closed sac, the conjunctival sac. This sac is 

 lined by the epiblastic layer, which remains as 

 the epithelium of the conjunctiva. The con- 

 nective-tissue portions of the lid are derived 

 from the mesoblast. 



The lachrymal duct is formed from the 

 lachrymal furrow (Fig. 19, A and B), a groove 

 lined with epiblast, extending from the eye to 

 the olfactory opening. This groove forms a 

 canal and becomes separated from the exter- 

 nal epiblast. 



To return and trace the development of the 

 cerebral vesicles. Their walls, as we have 

 seen, consist of a layer of involuted epiblast or 

 neural epiblast. The first marked change to 

 occur is that the posterior portion of the wall 



