94 THE ORIGIN OF VERTEBRATES 



Balfour, that this eye is an aborted eye, and that it cannot be 

 considered as a primitive type. Thus Balfour says : " Considering 

 the degraded character of the Ammoccete 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 

 Ammoccete. The development of such aborted eyes has as yet been 

 studied only in the Ammoccete, 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 life-time of the larval form. 



Kohl, who has especially studied degenerated vertebrate eyes, 

 discusses with considerable fulness the question of the Ammocoetes 

 eye, and concludes that in aborted eyes a retarded development 

 occurs, and this applies on the whole to Ammocoetes, " but with the 

 important difference that in this case the period of retarded develop- 

 ment is not followed by a stoppage, but on the contrary by a period 

 of very highly intensified progressive development during the meta- 

 morphosis," with the result that " the adult eye of Petromyzon 

 Planeri does not diverge from the ordinary type." 



Eeferring in his summing up to this retarded development, he 

 says : " Such reminiscences of embryonic conditions are after all 

 present here and there in normally developed organs, and by no 

 means entitle us to speak of abnormal development." 



The evidence, then, is quite clear that the eye of Petromyzon, 

 or, indeed, of the full-grown Ammocoetes, is in no sense an abnormal 

 eye, but simply that its development is slow during the animoccete 

 stage. The retina of Petromyzon was figured and described by 

 Langerhans in 1873. He describes it as composed of the following 

 layers : — 



(1) Membrana limitans interna. 



(2) Thick inner molecular layer. 



