182 



COMPARATIVE ANATOMY. 



it the outer wall of the vesicle so as to form a double-walled cup, 

 the secondary optic vesicle (Fig. 15lB). 



TJt 



Alii 



FIG. 15lA. DIAGRAM SHOWING THE MODE OF FORMATION OF THE PRIMARY 



OPTIC VESICLES (ABl}. 



VH, fore-brain ; V, V, ventricular cavity of the brain, which communicates freely 

 with the cavities of the primary optic vesicles at t+. 



2f 



FIG. 15lB. SEMIDIAGRAMMATIC FIGURE OF THE SECDNUAKY OPTIC VESICLE, AND 



OF THE LENS BECOMING SEPARATED OFF FROM THE El'IBLAST. 



IB, inner layer of the secondary optic vesicle, from which the retina arises ; t, point 

 at which the latter is continuous with the outer layer (AB), from which the pig- 

 ment epithelium arises ; H, remains of the cavity of the primary optic vesicle ; 

 L, lens which arises as a cup-shaped involution of the epiblast (E) ; *, point of 

 involution of epiblast to form the lens ; MM, mesoblastic tissue, which at J/ 1 , M l 

 grows in between the epidermis and the lens as the latter becomes separated off, 

 and which gives rise to the cornea as well as to the iris ; C, vitreous chamber of 

 the eye, between the lens and retina, which becomes later tilled by the vitreous 

 humour. 



The inner and outer walls of the cup then become fused 

 together, and from the former the sensory epithelium of the 



