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 Ammoccetes 
eye, and concludes that in aborted eyes a retarded development 
occurs, and this applies on the whole to Ammoccetes, “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.” 
Referring 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 Ammocetes, is in no sense an abnormal 
eye, but simply that its development is slow during the ammocc-te 
stage. The retina of Petromyzon was figured and described by 
Langerhans in 1873. He describes it as composed of the following 
layers :— 
(1) Membrana imitans interna. 
(2) Thick inner molecular layer. 
