THE DEVELOPMENT OF VERTEBBATA. 277 



several layers of which the outermost, i.e. nearest the 

 primary vesicle, gives rise to the rods and cones ; the inner 

 ones to the nerve-cells 

 and supporting ele- 



, r nm i i 



ments. The choroid 

 fissure and the groove 



that continues it along 



,. , ,, fi i4^( 



the optic stalk (fig. 145) 



become the way by 

 which all the nerve- 

 fibres growing out from 

 the nerve-cells make 

 their way to the first 

 cerebral vesicle, over Fjg m _ HoBIZONTAL SECTION TIIRO UOH BYK 

 whose surface they ex- AT T ATEB gTACEi 



tend to form the white 



matter of the optic tracts. The cells of the optic stalk 

 itself seem to degenerate and disappear after the fibres of 

 the optic nerve are formed. The choroid fissure eventually 

 closes up as far as it can, but its edges obviously cannot 

 completely fuse, because of the nerve fibres running out to 

 the optic nerve. The portion of the choroid fissure which 

 cannot be obliterated for this reason becomes the blind spot. 



12. History of Vertebrate Eye. The study of the development of 

 the Vertebrate eye (and it is in all essentials the same from the 

 lamprey to man) thus shows us how the two remarkable peculiarities 

 of the retina come about, viz. the position of the rods and cones, 

 more remote from the light than the nerve-cells ; and the presence 

 of a blind spot. It is worth while to pause for a moment and ask 

 why these peculiarities should be so. A moment's thought will 

 show that the particular layer which forms the rods and cones, 

 being next to the cavity of the primary vesicle, is really part of the 

 lining epithelium of the tubular central nervous system, and this is 

 shown by development to be part of the surface epiblast folded in. 

 Thus the rods and cones agree with the sensory cells of other sense- 

 organs in being part of the surface epiblast, but a part which has 

 reached its final position by a complicated series of changes. 



Many lowly animals, such as the sea-urchins, have the whole 

 epidermis sensitive to light, and have their nervous system largely 

 diffused beneath the epidermis. Starting from such a stage, if we 

 imagine both the nerve-cells and the sensitiveness of the epidermis 

 to be concentrated in the mid-dorsal region, and to be sunk first into 

 a canal and then formed into a tube we should arrive at the 



