274 



COMPARATIVE ANATOMY 



A chiasma, that is, a crossing of the fibres of the two optic 

 nerves, doubtless always occurs though not always freely exposed, 

 for it may retain a primitive position deeply embedded in the base 

 of the brain (e.g. Myxinoids, Dipnoans, and to a certain extent in 

 Petromyzon). In most Teleosts the optic nerves simply overlie 

 one another (Fig. 202, A), but in some of these Fishes (Clupea 

 (B), Engraulis), one nerve passes through a slit in the other, and 



r/i 



jf 



A B 



FIG. 201, A. DIAGRAM SHOWING THE MODE OF FORMATION OF THE PRIMARY 



OPTIC VESICLES (AR1). 



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

 the cavities of the primary optic vesicles at tt- 



B. SEMIDIAORAMMATIC FIGURE OF THE SECONDARY OPTIC VESICLE, AND OF 



THE LENS BECOMING SEPARATED OFF FROM THE ECTODERM. 



C, vitreous chamber of the eye, between the lens and retina, which later becomes 

 filled by the vitreous humour ; H, remains of the cavity of the primary optic 

 vesicle ; 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 pigment epithelium is formed ; L, lens, which arises 

 as a cup-shaped involution of the ectoderm (E) ; *, point of involution of 

 ectoderm to form the lens ; MM, mesoderrnic tissue, winch at J/ 1 , M 1 , grows 

 in between the outer ectoderm and the lens as the latter becomes separated 

 off, and which gives rise to the cornea as well as to the iris 



this condition of things is gradually carried still further in Reptiles, 

 until finally the fibres of the two nerves intercross in a very com- 

 plicated manner (c, D), giving rise to a sort of basket- work ; this is 

 finest and most delicate in Mammals, where its structure can only 

 be analysed by comparing a series of sections. 



To return to the further development of the eye, the ectodermic 

 thickening mentioned above becomes separated from the ectoderm, 

 sinks more and more into the interior of the optic vesicle, and is 

 differentiated to form the crystalline lens (Fig. 201, B). It is usually 



