280 MODE IN WHICH LIGHT ACTS ON THE RETINA. 



as the so-called rods and cones — that is, tlie photsesthetic bodies 

 which I have already described in the retina of the vertebrate 

 eye. But Leydig entirely loses sight of a. fact, which, if un- 

 explained, vitiates his conclusion as to the physiological 

 identity of the bodies in question. In the annulose or 

 molluscous eye, whether in its so-called simple or compound 

 form, the crystalline columns are directed, like the tubes of 

 so many telescopes, towards the object, the corresponding 

 nervous filaments passing to them from behind ; whereas the 

 crystalline rods of the vertebrate retina are directed away from 

 the object — that is, towards the back of the eye — are in contact, 

 in fact, with the choroid, while their nervous filaments are con- 

 nected to them in front — that is, between them and the object. 



On the other hand, if I am correct in holding that the 

 vertebrate eye is acted upon by those rays only which are 

 reflected from its choroidal surface, I have not only explained 

 physiologically why its retinal columns are reversed, but I am 

 legitimately entitled, as Leydig is not, to consider them as the 

 homologues of the crystalline columns of the annulose and 

 molluscous eye. 



But the teleological explanation of the opposite arrange- 

 ment of the corresponding structures in the vertebrate and 

 invertebrate eye, is, in the present phase of the science, in- 

 sufficient. The difference must be explained morphologically. 

 This explanation is afforded by the different modes in which 

 the vertebrate and invertebrate — that is, the simple and 

 compound — eyes are developed. 



In the compound eye the primordial ocular papilla or 

 convexity, which is only slightly protuberant, has its 

 cutaneous or superficial surface immediately converted into 

 the crystalline columnar structure, the individual columns of 

 which are connected with the filaments of the subjacent optic 

 nerve. The columns are all therefore directed to the object. 



The primordial cerebro-cutaneous spheroidal protuberance 



