238 DE WALTER II. GASKELL. 



development occurs, and this applies on the whole to Am- 

 mocoetes, " but ^ with the important difference that in this case 

 the period of retarded development is not followed by a stoppage, 

 but on the contrary by a period of very highly intensified 

 progressive development during the metamorphosis," 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 Ammocoete stage, and taking into consideration the affinity 

 of the Cephalaspids with Ammocoetes, we may, I think, take it 

 for certain that such eye will more closely represent the lateral 

 eye of the Cephalaspids than any other known vertebrate eye. 

 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. 



(3) Optic fibre layer. 



(4) Thick inner nuclear layer. 



(5) Pecuhar double-layered ganglionic layer. 



(6) External molecular layer. 



(7) External nuclear layer. 



(8) Membrana limitans externa. 



(9) Layer of rods. 



(10) Pigment epithelium. 

 He points out especially the peculiarity of layer (2) (2, fig. 4), 

 the inner molecular, in which two rows of nuclei are arranged 

 with great regularity, the one row closely touching the membrana 

 limitans interna, the other at the inner boundary of the middle 

 third of the molecular layer. Of these two rows of nuclei, he 

 describes the innermost as composed almost entirely of large 



1 Op. ciL, p. 269. '^ ^(^; V- 265. 



3 Langerhans, Untersuch. ii. Petromyzon Planeri, Freibury i. B., 1873. 



