64 



BLIND VERTEBRATES AND THEIR EYES. 



2- J 



C 



THE EYES OF ZYGONECTES NOTATUS. 



Of the eyes of a number of species of normal fishes, namely those of Cyma- 

 togaster aggregates, Carassius auratus, Amciurus sp., Coregonus sp., and Zygonectes 

 riotatus examined, I shall briefly describe the eyes of but one. 



Zygonectes notatus (Rafinesque) was selected for comparison, since it is a 

 member of the Cyprinodontidae, a family closely related to the Amblyopsidae. I 

 am not aware that this species has any advantage over 

 other species of the family. It has large, well-developed 

 eyes, that we may assume to be fully and normally devel- 

 oped. The material examined was alcoholic. It had been 

 preserved by simply placing in alcohol without any intention 

 of future histological examination, but the structures were all 

 well preserved for making out the horizontal relations of the 

 single and twin cones. The protoplasmic and nervous 

 processes of the cells were of course not brought out as 

 with Golgi's method. 



A specimen 38 mm. long had the eye 2.24 mm. in 

 length, 2 mm. in vertical diameter, 1.12 mm. from axis of 

 optic nerve to front of iris, 1.6 mm. from axis of optic nerve 

 to front of cornea; lens 0.96 mm. in diameter; pigment 

 layer measures 56 /i. ; outer nuclear layer, 36 /x ; outer reticu- 

 lar, 4/i.; tangential cell layer, 9 /a; inner nuclear, 40 /*; inner 

 reticular, 52 /x; ganglionic layer, 12 /a; optic-fiber layer, 28 ft; 

 total thickness of retina, 237 ti. 



The regularity of arrangement of single and twin cones 

 is very striking. The basal part of the single cones con- 

 tains refractive granules increasing in size outward where 

 the series ends in a lenticular vacuolated body separating the 

 granular from the distal part of the rod. The twin cones are 

 all without granulation. This marked difference between 

 the two enables one to distinguish between them at a glance 

 in tangential sections. The twin cones are arranged in 

 series in such a manner that the axes joining the cones in any 

 neighboring series are at right angles to each other, while in 

 every alternate series they extend in approximately the same 

 or parallel directions. The single cones alternate in all 

 directions with twin cones (fig. 24 b). 



The outer nuclei are irregular, compressed, and elongate, 



The outer molecular layer has 

 an irregular outer boundary produced by the process extend- 

 ing toward the outer cells. The inner nuclear layer is divided into an outer layer 

 of small bipolar cells and an inner layer of larger, more coarsely granulated 

 spongioblast^' (ills. When any breaks occur in the retina, owing to mechanical 

 or chemical causes, they usually occur between these outer bi-polar and inner 

 spongiose cells of the inner nuclear layer. 



Fir,. 24. (<i) Section through Retina 

 of Zygorucus nolatus. ,- 

 (4) Cone Pattern o( Same IOrmmg tWO dlStlllCt laVerS. 



