THE ORGANS OF THE OUTER GERM-LAYER. 489 



Finally, as far as regards the development of the lachrymal tubules in Birds 

 and Mammals, BORN and LEGAL refer the upper tubule to the proximal 

 part of the epithelial ridge, and maintain that the lower one buds out from 

 the upper. EWETSKY, on the contrary, declares that the proximal end of the 

 epithelial ridge expands at the inner angle of the eye, and becomes divided 

 by the ingrowth of underlying connective tissue, and metamorphosed into the 

 two tubules, so that both arise from a common fundament. 



SUMMARY. 



1. The lateral walls of the primary fore- brain vesicle are evaginated 

 to form the optic vesicles. 



2. The optic vesicles remain united by means of a stalk, the 

 future optic nerve, with that part of the primary fore-brain vesicle 

 which becomes the bet ween -brain. 



3. The optic vesicle is converted into the optic cup through the 

 imagination of its lateral and lower walls by the fundaments of the 

 lens and vitreous body. 



4. At the place where the lateral wall of the primary optic vesicle 

 encounters the outer germ-layer, the latter becomes thickened, then 

 depressed into a pit, and finally constricted off as a lens-vesicle. 



5. The cells of the posterior wall of the lens-vesicle grow out into 

 lens-fibres, those of the anterior wall become the lens-epithelium. 



6. The fundament of the lens is enveloped at the time of its 

 principal growth by a vascular capsule (tunica vasculosa lentis), which 

 afterwards entirely disappears. 



7. The membrana capsulo-pupillaris is the anterior part of the 

 tunica vasculosa lentis and lies behind the pupil. 



8. The development of the vitreous body causes the choroid 

 fissure. 



9. The optic cup has double walls ; it consists of an inner and an 

 outer epithelium, which are continuous with each other at the open- 

 ing of the cup, which embraces the lens, and at the choroid 

 fissure. 



10. Mesenchymatic cells from the vicinity grow in between the 

 lens and the somewhat closely applied epidermis to form the cornea 

 and DESCEMET'S membrane, the latter being separated from the 

 tunica vasculosa lentis by a fissure, the anterior chamber of the 

 eye. 



1 1 . The optic cup is differentiated into a posterior portion, within 

 the territory of which its inner layer becomes thickened and con- 

 stitutes the retina, and an anterior portion, which begins at the ora 



