No. 501] THE ORIGIN OF VERTEBRATE EYES 607 



Viewed from this standpoint the optic cups of amphi- 

 oxus are not without interest. They can be seen well in 

 any transverse section of the nerve tube of this animal 

 (Fig. 2) provided the section is not taken from a part 

 of the body too far toward the anterior end. In such a 

 section the cups will be found close to the central canal. 

 Each cup is made up of a concave pigment cell in the 

 hollow of which is a fan-shaped cell. The expanded end 

 of this fan-shaped cell fills the pigment cup while the 

 handle of the fan tapers off into a nerve fiber. The 

 whole structure, as Hesse observed, has a most striking 

 resemblance to the simple direction eye of certain planar- 

 ians, and it is difficult to resist the conclusion that amphi- 

 oxus is an animal whose nerve tube is richly provided 

 with a simple type of direction eye. 



If these bodies in the nerve tube of amphioxus are 

 direction eyes, their positions are not without significance. 

 On examining a number of transverse sections in which 

 they occur, it will soon be recognized that their arrange- 

 ment is any thing but regular. Most of them are directed 

 either dorsally or ventrally, but almost any position may 

 be assumed. Though they lie near the central canal, 

 they are not arranged in reference to it with any de- 

 gree of uniformity. Some point away from it, others 

 toward it. 



The lack of uniform arrangement shown by these visual 

 cups is of considerable importance in relation to Bal- 

 four's theory of the origin of the retina. If this theory 

 is correct, the inversion of the retina is dependent upon 

 a rigid orientation of the sensory cells which is supposed 

 to have been retained absolutely from their place of 

 origin of these cells in the external skin, through every 

 step in their migration to their final position in the com- 

 pleted retina. If once in the course of this migration 

 orientation is lost, the mechanism fails. Since amphioxus 

 is the only animal that represents well an intermediate 

 stage in this process, and since in amphioxus the orienta- 

 tion is lost, it seems to me that Balfour's explanation 



