1888.] ^ MICROSCOPICAL JOURNAL. 233 



Parietal Eye in Yoimg Petromyzon, Adult Petromyzon, and Myxine.* 



By J. BEARD. 



In ammocoetes, the larval form of Petromyzon, the parietal eye is hemi- 

 spherical in shape, the lens being nearly flat and the retina and nerve-gangli- 

 onic layer being curved. The eye is overlaid by connective tissue and the 

 membranous parietal bone, which in turn is covered by an epidermis of 

 columnar cells. The lens portion of the eye is slightly enlarged in the centre. 

 Its cellular structure is not perfectly discerned. The retinal portion is com- 

 posed of very obvious rod-shaped structures which have their free ends toward 

 the cavity of the eye opposite to the pattern of the paired eyes of vertebrates. 

 The deeper portions of the rods are lost in two rows of rounded nucleated 

 bodies believed to be nerve-ganglion cells. In some species the lower ends 

 of the rods are surrounded with abundant black pigment ; in others they are 

 almost destitute of it. 



In the adult Petromyzon the position of the organ is marked externally by 

 a whitish spot in the skin due to the absence from that spot of the usual black 

 pigment of the epidermis. The parietal eye usually lies in a deep depression 

 and is pigmented, but if it is not pigmented in its retina it then lies in a shal- 

 low depression, or the depression is wholly absent. The organ in the adult, 

 as in the young Petromyzon, lies beneath the membranous I'oof of the skull. 

 It has besides the rods, as in Myxine, organs like the cones of the paired eye. 



In Myxine, though from very poor material, the author describes a flattened 

 two-liiyered organ, connected by a stalk with the brain. The hinder wall of 

 the organ is a retina in which I'ods are distinguished and figured. The nuclei 

 of these cells lie close to the cavity of the eye (in Petrom3'zon they are at the 

 opposite end of the rods). In the specimens examined no retinal pigment 

 could be found. 



The writer had hoped to determine some points in the phylogeny of this 

 remarkable organ by its study in the young Petromyzon, but he proves very 

 little. He cannot think that it is one of the series of lateral sense organs as 

 are the paired eyes and ears. He also regards with disfavor the idea of 

 vSpencer, that it may have been derived from the eye of the larval truncate, 

 because the truncate seems well proven to be only a degenerate ofishoot of a 

 possible ally of the original vertebrate stock. The structure of the retina 

 proves the eye to be unlike the paired type in its mode of development. He 

 thinks, however, that the eye has developed in connection with the paired 

 eyes, because the fibres from it run to the optic thalamus, and because in 

 Petromyzon both rods and cones form the terminal organs. 



A speculative suggestion as to the origin of the parietal eye is ventured :- 

 That the paired eyes were once structures opening dorsally on the surface ; 

 that the parietal eye did not at that time exist : later the paired eyes were shut 

 in together with the rest of the central nervous system, and the paired eyes 

 were beneath the skin. At that time the paired eyes would receive light both 

 through the side skin and through the median suture of closure. The side 

 portions of the outer skin became set oft^ as the lenses, and beneath them the 

 sensory epithelium as the retinas of the paired e3'es. The part of the sensory 

 epithelium beneath the median line remained for a time as the pineal eye. 

 The remainder of the former sensory epithelium aborted. This utilization 

 of part of the sensory epithelium of the paired eyes is paralleled in the organ 

 of Jacobson in Reptiles, and which arises from a portion of the olfactory- 

 epithelium. 



* Abstract from a paper in the Quarterly yournal of Microscopical Science, 1888 . 



