42 



VERTEBRATE PHOTORECEPTORS 



Typically the inner segment of the cone, which is some- 

 what barrel- or keg-shaped, possesses an ellipsoid and a 

 myoid element. Frequently in addition, there is present a 

 highly refractive globular structure called the paraboloid, 

 and in some classes an oil globule (Figure 30). The myoid 

 element in some forms (fishes, amphibians, birds) is con- 

 tractile; hence its shape and length varies. It contracts in 

 the light and lengthens in the dark. According to Arey 

 (1915, 1916a) the maximum ratio of an extended to a con- 

 tracted myoid in fishes may be 10 : 1. The ellipsoid or 

 lentiform body lies next to the external segment (except in 

 cones with oil globules). It too varies somewhat in shape 

 and size. In mammals it occupies two-thirds of the volume 

 of the inner segment. It is said to be absent in some reptiles. 

 A paraboloid is found in some amphibians, reptiles, and in 

 the accessory component of double cones 

 (Figure 30, D and E). It is absent in the 

 cones of teleost fishes, mammals, anthropoids, 

 and man. 



Double cones (Figure 30, D) occur in all 

 retinas except in mammals. They consist of 

 two components or members (a principal and 

 an accessory part) . The principal (chief or far) 

 cone has a long narrow myoid, a long ellip- 

 soid, and an oil globule, there being no parabo- 

 loid. The accessory (or near) cone, which 

 is much broader and shorter than the principal 

 cone, has a typically short myoid, a parabo- 

 loid, a granular ellipsoid, but no oil drop. In 

 addition to double cones, there occurs also 

 the so-called twin cones in which each mem- 

 ber is of equal size (Figure 31). In fishes they 

 are said to occur in forms with well developed 

 eyes and never in nocturnal fishes with small 

 eyes. According to Wunder (1926) this makes for better 

 vision. The double and twin cones presumably arise from 

 the fusion of pre-existing single cones. Levi (1901), however, 



Fig. 31. Twin 

 cone of the perch. 

 XIOOO. (Re- 

 drawn from Arey, 

 1928, Special Cy- 

 tology, V. 2, Paul 

 B. Hoeber, New 

 York.) 



