42 H. M. BERNARD. 



may find the latter frequently separated by what appear to be 

 long vacuoles, which cannot yet be called cones, but which I 

 take to be a lingering of the earlier undifferentiated condi- 

 tion. Fig. 16 (from an eye '75 mm. diameter) shows the 

 appearances ; the sacs can there be traced to definite nuclei 

 which occupy the position of the nuclei of very young cones 

 (see below on the positions of the nuclei). Such long open 

 sacs as these can only be regarded as the last remains of the 

 condition just described for still younger eyes. In fig. 12, A, 

 which is from an eye 1*12 mm. in diameter, a collapsed sac 

 can be seen, which shows that at any time in a rapidly gi'ow- 

 ing eye the embryonic sac-like extension may reappear. As 

 soon as the rods and cones are multiplied sufiiciently to form 

 a compact layer, each new vacuole forcing its way down can 

 presumably only appear in the form shown in fig. 4, c^, with 

 a slight proximal and a larger distal swelling, the intervening 

 portion being squeezed to a thread. 



I had hoped to give, as a second part of this paper, an 

 account of what I have been able to unravel of the details of 

 this process of transformation from the early sac stage to the 

 definitive rod stage, and also of the minute structure of the 

 rods themselves ; but other duties have intervened, and the 

 completion of the second part must be postponed for a few 

 months. 



I will, however, here append one more argument in favour 

 of my conclusion that, in the Amphibia, cones are merely 

 stages in the formation of new rods. It is derived from a 

 comparison of the positions of the nuclei. 



The nuclei of fully developed rods are invariably found in 



various degrees of protrusion beyond the memb. limitans ext. 



This protrusion is usually very pronounced in the Urodeles 



and in the toads, but less so in the frog. In younger rods, 



Schwalbe's rods, the nuclei seldom if ever project beyond 



this line, while the cone nuclei are still further away and 



nearer to the outer reticular layer the younger they are.^ 



' This is not so rigidly the case in youug eyes, in which the raultiplicatiou 

 of elements is very rapid. 



