172 ELEMENTARY BIOLOGY. [CHAP. I. 



///. Across the head of /. (15 18 days); transversely 

 to the long axis, to pass through the eyes. 



a. The epidermis ; forming a continuous investment 

 for the whole ; thickened below and produced 

 into two inverted cup-shaped folds (suckers). 



ft. The brain ; dorsal and elongated, pigmented 

 and enclosing a spacious central cavity ($rd 

 ventricle). 



y. The notochord and archenteron ; occurring in 

 order of succession below ft. 



8. The developing retina; on either side a cup- 

 shaped outgrowth of the brain (optic cup), con- 

 nected with the same by a narrow neck (optic 

 stalk) visible only in a few sections. Examine 

 the cup and note its thick outer wall ; its thin 

 inner wall; its central cavity, continuous with 

 that of the 3rd ventricle. 



e. The lens ; visible either as a thickening or invo- 

 lution of the epiblast, immediately external to 8. 



. Compare similar sections through the eye of a 

 more advanced larva. Examine under a high 

 power. 



a. The optic cup ; its central cavity becoming ob- 

 literated, its inner wall densely pigmented (pig- 

 ment epithelium ; cf. Sect. M. 1 2 k}. 



ft. The lens ; completely constricted off from the 

 epidermis and enclosing a central cavity; its 

 thin outer wall ; its thick inner wall. 



y. The remaining constituents of the eye ; seen, 

 for the most part, in course of differentiation 

 from the surrounding mesoblast. 



