ON THE OEIGIN OF VEKTEBRATES. 237 



The Bctina of the Lateral Eye of Ammocostes. 



As in the case of all other organs, it follows that if we are 

 dealing here with a true genetic relationship, then the lower 

 we go in the vertebrate kingdom the more nearly ought the 

 structure of the retina to approach the arthropod type. It is 

 therefore a matter of intense interest to determine the nature 

 of the retina in Ammocoetes in order to see whether it differs 

 from that of the higher vertebrates, and if so, whether such 

 diff'erences are explainable by reference to the structure of the 

 arthropod eye. 



Before describing the structure of this retina it is necessary 

 to clear away a remarkable misconception, shared among others 

 by Balfour, that this eye is an aborted eye, and that it can- 

 not be considered as a primitive type. Thus Balfour says : ^ 

 " Considering the degraded character of the Ammocoete eye, 

 evidence derived from its structure must be received with 

 caution," and later on,^ " the most interesting cases of partial 

 degeneration are those of Myxine and the Ammocoete. The 

 development of such aborted eyes has as yet been studied only 

 in the Ammocoete, in which it resembles in most important 

 features that of other Vertebrata." 



Again and again the aborted character of the eye is 

 stated to be evidence of degeneration in the case of the lamprey. 

 What such a statement means, why the eye is in any way to be 

 considered as aborted, is to me a matter of absolute wonderment : 

 it is true that in the larval form it lies under the skin, but it 

 is equally true that at transformation it comes to the surface 

 and is most evidently as perfect an eye as could be desired. 

 There is not the slightest sign of any degeneration or abortion, 

 but simply of normal development, which takes a longer time 

 than usual, lasting as it does throughout the lifetime of the 

 larval form. 



Kohl,^ who has especially studied degenerated vertebrate 

 eyes, discusses with considerable fulness the question of the 

 Ammoccetes eye, and concludes that in aborted eyes a retarded 



1 O2K cit., p. 406. - Id., p. 409. 



•^ Kohl, "Rudimentare Wirbelthieraugen," EiUiotlieca Zoolog., Leuckart and 

 Chun, vol. 4 and vol. 5. 



