FIG. 



FIG. 389. 



B 



Section through head 

 of ten-day rabbit em- 

 bryo, exhibiting primary 

 optic vesicle (O) pro- 

 truding from fore-brain 

 (B) and coming in con- 

 tact with surface ecto- 

 derm (e) : in, surround- 

 ing mesoderm. 



Section through develop- 

 ing eye of eleven-day rab- 

 bit embryo : B, fore-brain 

 connected by stalk with 

 optic vesicle (o), whose 

 anterior wall is partly in- 

 vaginated ; /, thickened and 

 depressed lens-area. 



THE EYE AND ITS APPENDAGES. 373 



the anterior segment of the primary optic vesicle undergoes an 

 important invagination, whereby the front wall of the sac is pushed 

 into the cavity of the vesicle 

 until eventually the anterior 

 and posterior walls are in 

 apposition and the included 

 cavity is largely obliterated. 

 The new space within the 

 indented anterior walls of the 

 sac constitutes the second- 

 ary optic vesicle and corre- 

 sponds to the later vitreous 

 chamber. These important 

 changes probably are not en- 

 tirely attributable to the me- 

 chanical influence exerted by 

 the developing lens-sac on the 

 closely-applied optic vesicle, 

 but must be referred also to 

 deeply-lying formative forces. 



The invagination of the optic vesicle is not confined to the 

 anterior pole, but takes place likewise along the under side of the 

 sac as well as along the optic stalk ; in consequence the vesicle is 

 imperfectly closed below, the 

 cleft, or choroidal fissure, 

 thus established affording an 

 entrance for the surrounding 

 mesodermic tissue which takes 

 part in the production of the 

 primary vascular structures oc- 

 cupying the vitreous chamber. 

 The relations of the parts to 

 the fissure are well shown in 

 frontal sections, where the cleft 

 appears as a conspicuous break 

 in the continuity of the walls of 

 the vesicle. 



The Retina. The layers of 

 the optic vesicle very soon ex- 

 hibit marked difference in their 



Sagittal section through developing eye of eleven- 

 and-a-half day rabbit embryo, exhibiting choroidal 

 fissure (C) through which mesodermic tissue (m) 

 reaches interior of secondary optic cup : o, i, outer 

 and inner layers of optic vesicle ; /, lens-sac. 



rate of growth, since the an- 

 terior depressed lamina rapidly overshadows the posterior layer by its 

 much greater thickness and more active proliferation. The posterior 

 wall becomes reduced in thickness, owing to the increase in the size 



