EYES OF THE POLISTOTREMA STOUTI. 61 



EYES OF THE CYCLOSTOME POLISTOTREMA STOITI. 



The eyes of this myxinoicl of the Pacific coast were examined by Allen and by 

 Stockard. Allen found that they show a very primitive structure, which is in 

 reality the result of a complex process of degeneration. The eyeball is found em- 

 bedded in a mass of fat about three times its size. In one case, the eye was found 

 to lie some distance beneath the outer surface of the mass of fat. Normally, how- 

 ever, the corneal surface lies on a level with the surface of the fat and is often 

 flattened to form a rather extensive free surface. No eye muscles nor traces of 

 such were discovered. No oculomotor nerves were found. No traces of them 

 are discoverable in embryonic life (Kupffer). The choroid and sclerotic coats 

 are represented by a very thin layer of unpigmented, non-vascular connective 

 tissue without any appreciable distinction between corneal and sclerotic portions. 

 The retina remains in the early condition of an optic cup, the outer layer (pigment 

 layer) not being fused with the remaining layers. All specimens showed the layer 

 in question to be widely separated from the bulk of the retina. This pigment 

 layer is composed of a single layer of cubical cells devoid of pigment as far as 

 could be ascertained. A layer corresponding to that of the rods and cones in higher 

 vertebrates is clearly present. The nuclei of these structures (outer nuclear layer) 

 are strikingly well developed and regularly arranged. Certain characteristic cells 

 of the inner nuclear layer could be readily made out. The ganglionic layer is 

 represented by cells scattered irregularly throughout the inner reticular layer. 

 Fibers from these last-named cells can be traced in a more or less direct course to 

 the optic nerve. The outer rim of the optic cup is in many cases differentiated in 

 such a manner as to suggest a rudimentary iris. A structure unmistakably like 

 an iris was found in one specimen examined. The cellular structure of this rudi- 

 mentary iris is almost identical with that of the pigment layer. No indications 

 of muscle fibers or pigment are to be seen. Certain deeply staining coagula within 

 the optic cup give evidence of a vitreous body. Some large, clearly-marked cells, 

 probably those of the vitreous body, are found attached to the surface of the retina. 

 Evidences of a choroid fissure are to be seen in the fact that the ventral part of 

 the retina is thinner than the dorsal in almost all specimens. In one case the 

 choroid fissure was found to persist. The most striking feature, however, is the 

 extreme variation. The optic nerve enters the eye at various angles. Variation 

 occurs in all parts of the eye and is especially notable in the measurements of the 

 thickness of the retina and the dimensions of the eye as a whole. 



Stockard found that the lens-bud results from a contact of only a portion of 

 the optic cup with the ectoderm. This structure continues to develop for a time 

 until, in an embryo considerably more advanced and measuring 15 mm. in length, 

 one sees the lens-bud with a slight indication of a constriction about the periphery 

 of its area of union with the ectoderm, as if it were preparing to pinch off. Here 

 the progressive development of the lens ceases and degeneration begins. It soon 

 disappears entirely. He considers the cessation of development in the lens due 

 to the absence of a durable contact with the optic cup upon which lens formation 

 is directly dependent. 



