422 W. H. GASKELL. 



through the pineal eye of a full-grown Ammocoetes, the whole 

 head of which was placed in osmic acid soon after it was caught. 



These sections show very clearly the arrangement of the 

 pigment in the eye, the shape and position of the eye, the 

 course and relations of its nerve, and the manner in which 

 its supposed central cavity is filled up with masses of definite 

 protoplasmic material. 



]. The Pigment Layer and Nerve-end Cells. — With 

 respect to the pigment, we see that Wiedersheim (19) and 

 Ahlborn (3) describe the eye as containing, in all cases, intensely 

 white pigment, while Beard (20) asserts that in all his speci- 

 mens of the eyes of Ammocoetes pigment was not present 

 except in three sections which were given to him and came 

 from another locality ; in these specimens the pigment was 

 black, not white. He concludes, therefore, that pigment is 

 only rarely present in the eyes of Ammocoetes Planeri, 

 that the white pigment of Ahlborn and Wiedersheim if it is 

 present is dissolved away in the process of preparation ; and he 

 claims to have discovered the presence of black pigment. 



By the careful study of the pineal eye I am able to abso- 

 lutely clear up these apparent discrepancies of fact, and to 

 show that Ahlborn is entirely right in his interpretation, while 

 Beard has misinterpreted what he saw. The eye of every 

 Ammocoetes, without exception, presents an intensely white 

 appearance, due to the presence of white pigment as described 

 by Ahlborn ; never in one single instance have I seen a speci- 

 men free from this glistening white substance. At first, 

 specimen after specimen which I cut presented on section lines 

 and markings of apparently black pigment without exception, 

 so that I was utterly unable to understand Beard's assertion 

 that the eye in Ammocoetes was only rarely pigmented ; at 

 the same time it was difficult to understand how such a white, 

 glistening little mass could be so full of what was apparently 

 intensely black pigment. Afterwards I began to find that 

 some of my preparations were entirely free from pigment, and 

 looking through the whole series which I possessed, it was 

 immediately evident that all those which were killed with 



