No. 501] THE OBIGIN OF VERTEBRATE EYES 605 



nervous system and the inverted condition of its receptive 

 cells. 



The most obvious objection to this view is that the 

 retina must be supposed to be a functional organ during 

 this whole process of migration and inversion. But this 

 objection, as others have already pointed out, is not 

 serious, for if we imagine these processes to take place 

 in animals like certain pelagic tunicates or even amphi- 

 oxus, the transparency of the body would be such as to 

 offer no hindrance whatever to the continuous action of 

 such an organ even when in its deepest position. From 

 this standpoint, therefore, there is no serious objection 

 to the theory as already outlined. 



The various adherents of this theory have differed con- 

 siderably as to the details of its application. The gen- 

 eral account of it already given is the particular form 

 which was advocated by Balfour (1881) and which has 

 since been revived by Jelgersma (1906). Balfour makes 

 no mention of any living animal that might be taken to 

 represent, according to this theory, a stage in the evolu- 

 tion of the eye. Von Kennell (1881) designated the 

 annelids as probable ancestors and attempted to show 

 how the external eyes of these worms could be converted 

 into the type of eye found in the vertebrates. Jelgersma 

 on the other hand has accepted amphioxus as an inter- 

 mediate form and has based his description of the evolu- 

 tion of the eyes on the conditions found in this animal. 

 These three investigators agree in believing that the 

 vertebrate eyes have arisen from optic organs that were 

 once on the exterior of an ancestral form. Lankester 

 (1880) and Boveri (1904), however, do not place the 

 origin of the eye in so remote a region but believed that 

 it arose in the central nervous system in such a place as 

 is now occupied by the eye of the ascidian larva (Lankes- 

 ter) or the numerous eyes of amphioxus (Boveri). In 

 this way these two investigators abandon some very sig- 

 nificant parts of the Balfour theory but in other respects 

 they follow it very closely. 



