182 COMPARATIVE ANATOMY. 



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

 the secondary optic vesicle (Fig. 15lB). 



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 tt. 



Fia. 151s. SEMIDIAGRAMMATIC FIGURE OF THE SECONDARY OPTIC VESICLE, AND 



OF THE LENS BECOMING SEPARATED OFF FROM THE EPIBLAST. 



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 (AE), 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 M l , 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 filled 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 



