34 H. M. BERNARD. 



the diagram^ fig. 4, became as it were passive, i.e. ceased to 

 distend its basal section by any furtlier extrusion of matter, 

 the lateral pressure of the basal sections of younger and still 

 actively growing cones could hardly fail to force out distally 

 the material from the basal portions of its inner limb. We 

 thus have, on the one hand, zones of pressure due to the dif- 

 ferent thicknesses of the sections of the cones, while, on the 

 other, we have the outer limbs of the developing rods dis- 

 tending and lengthening in a direction opposite to that in 

 which the cones are forcing their way. It seems but natural 

 that the lengths of the growing outer limbs would be found 

 to have some correspondence with the different zones of 

 lateral pressure through which they have to force a passage. 

 It will be remembered that we have already assumed the 

 existence of this lateral pressure as necessary to account for 

 the shape of the cones themselves. The rods form a compact 

 layer into which fresh elements as conical wedges force their 

 way, their continued intrusion keeping the layer permanently 

 compact. In spite of the frequent spaces which appear in 

 microscopic sections between the rods, the evidence from 

 the best preserved material leaves little doubt but that, in 

 life, these retinal elements are packed together as tightly 

 as the intruding cones can wedge them. Many of the clear 

 spaces are due to rods falling out of the sections, others 

 perhaps to shrinkage, others again to the presence of large 

 vesicles filled with fluid, the walls of which are so delicate 

 as to be very seldom preserved (see below, p. 42, and fig. 16). 

 Our comparison of the rods and cones in the retina of the 

 adult frog has brought us to the conclusion that the latter, in 

 the frog's eyes at least, are not elements with any special 

 sensory function, as was maintained by the earlier investi- 

 gators, but that they are simply stages in the development of 

 fresh rods ; and that, in addition to their potential value as 

 future rods, thej serve, even in their earliest stages, to keep 

 the layer of the rods compact. 



cones shown in van Genderen Stort's figure reproduced in * Quain's Anatomy/ 

 10th edit., iii, part ill, p. 48, 1894. 



