THE SENSE ORGANS 407 



Among cephalochordates the so-called eye of Amphioxus is a pigmenl 

 spot located at the anterior end of the nerve cord. There is obviously 

 very little resemblance between such a structure and the eye of a verte- 

 brate. The reasons for calling it an eye are that pigment is associated 

 with eyes, and that this special pigment spot is associated with the anterior 

 expansion of the nerve cord, which is generally homologized with the fore- 

 brain of vertebrates. On the basis of such sUght resemblance, it is 

 scarcely possible to derive the paired vertebrate eyes from this unpaired 

 pigment spot. 



In the floor of the nerve cord of Amphioxus, throughout its length, are 

 photoreceptors partially enclosed by pigment capsules. Since removal 

 of the pigment spot from the so-called brain of Amphioxus does not affect 

 its response to light, it is assumed that the true light-recipient organs of 

 this animal are these photoreceptors of the nerve-cord. 



The study of invertebrates reveals that their eyes have all the histolog- 

 ical elements of the paired eyes of vertebrates, but never in the same 

 combinations. Some invertebrates have photoreceptors in the form of 

 rods and cones associated with pigment spots and with the brain. But 

 vertebrates alone have eyes with an inverted retina formed as an out- 

 growth of the brain wall, and surrounded by a mesenchymatous capsule. 

 Comparative anatomy throws little, if any, light upon the past history 

 of vertebrate paired eyes, since the eyes of the cyclostomes are in all essen- 

 tials like those of the highest vertebrates. Eyes, therefore, appear to 

 spring into existence full-formed, and we are compelled to draw phylo- 

 genetic conclusions from the facts of ontogenesis. 



These facts appear to justify the conclusion of Ray Lankester (1880) 

 that paired eyes of vertebrates arise from paired pigmented depressions 

 in the anterior part of the neural plate. It has been asserted that the 

 parietal eye has a similar paired origin, from pits anterior and lateral to 

 those which form the paired eyes; and the conclusion has been drawn that, 

 when the neural plate was converted into a neural tube, the unpaired eye 

 was formed by the fusion of the lateral paired pits, which subsequently 

 grew out as a stalked vesicle. Thus the parietal eye looks upwards, while 

 the paired eyes, bulging laterally from the brain wall, are receptors of 

 light from the sides and below. In most vertebrates, the median eye 

 degenerates, but the paired eyes enlarge and become the definitive organs 

 of vision. 



It is fairly easy to imagine the conditions which led to the lateral 

 outgrowth of the paired eyes. Among the factors involved was presum- 

 ably the enlargement of the head and the recession of the brain from the 

 surface. Less Hght, consequently, would reach the photoreceptors in the 

 brain wall. When the skin became pigmented, eyes in the brain wall 

 would be useless. 



