STUDIES IN THE HETINA. 39 



Young Forms. — Of these I have examined different 

 stages in the development of frogS; toads^ and salamanders. 

 All confirm very strongly the conclusions we have come to 

 above, that the cones in the Amphibia are merely stages in 

 the development of the rods. 



In eyes under 0'5 mm. in diameter the conditions fre- 

 quently require special study, and will be briefly referred to 

 below. We will confine our attention for the present to 

 eyes over 1 mm. in diameter, in which the definitive conditions 

 are becoming apparent. In such eyes we are at once struck 

 by the immense numbers of cones in all stages of protrusion, 

 far more in comparison with the number of the rods than in 

 the adult eye, in which the proportions are reversed. Fig. 11 

 is that of a young salamander (1*6 mm. eye diameter). Fig. 

 12, A, B, c, are from a toad tadpole (1*12 mm. eye diameter). 



Another phenomenon of these young eyes is the great 

 number of long, vacuolated inner limbs of the rods, the 

 outer limbs of which are short, and too often obscured by 

 pigment. This condition is not to be confused with that 

 which will presently be noted as being common to eyes under 

 0"5 mm. in diameter, which it may superficially resemble; 

 for these long, apparently empty inner limbs are sometimes 

 so numerous that it is difficult, in sections, to make out the 

 connections at all. Fig. 13 shows the appearance referred to 

 in a toad tadpole (ca. 1*5 mm. eye diameter). Fig. 14 is from 

 a young salamander (1*6 mm. eye diameter). The elements 

 drawn are some way from the centre of the eye, where the 

 vacuoles were much longer and narrower, but hardly any 

 section cut them longitudinally, and the result was too con- 

 fused to di-aw. Reference to the diagram (fig. 8) will fully 

 accouut for these long inner limbs, which are not character- 

 istic of adult eyes, although existing in small numbers. 

 Those shown in the diagram (fig. 8), for instance, were found 

 in about three thin sections of the retina of an adult axolotl. 

 We see from this diagram that any large number of cones 

 must naturally mean a correspondiugly large number of 

 immature rods, with long vacuolar inner limbs. 



