AND THE MOEPHOLOGT OF THE EYE IN INSECTS. 415 



The lens is not unlike the crystalline lens of a vertebrate. It is albuminous, or at 

 least gives a characteristic yellow with nitric acid. 



Beneath the lens a fusiform spindle is seen, very similar to the spindle of the com- 

 pound eye. This has a retinula beneath it consisting of a small fasciculus of bacilla. 

 Several of these eyes are united by a common optic nerve, like the stemmata of insects, 

 and three usually form a group. 



I believe that the spindle in this form of eye has the same function as that which I 

 have assigned to it in the compound eye. 



A comparison of the figures given by Dr. Grenadier * of the eyes of Porcellio and of 

 Sialis larva with those of the ocelli of caterpillars which accompany this paper, will show 

 how close the relation of these structures is to each other. On the other hand the 

 absence of a subcorneal lens and spindle, as well as the arrangement of the retinal 

 elements, in the simple ocelli of insects and arachnids, show that these organs are formed 

 on a type which differs essentially from that exhibited by the compound eye. 



1 believe, however, that the columnar cells immediately beneath the cornea, the 

 vitreous of Dr. Grenadier, represent the dioptron ; they are undoubtedly of hypodermic 

 origin, and are separated from the retina by a fibrous membrane which apparently 

 corresponds to the membrana basilaris of the compound eye. Dr. Grenadier figures 

 and describes this membrane t in a species of Salticus, and traces the fibres of which it 

 is composed, from the outer ends of the bacilla of the retina to a number of nuclei 

 situated in a sinus which surrounds its margin. I have made some remarkably good 

 sections of the eyes of Salticus scenicns, in which it is quite easy to see that no such 

 connexion exists between these structures. 



In some of these sections the fibrous membrane has completely separated from the 

 bacilla, just as the membrana basilaris separates from the retina in the compound eye. 



The sinus (Is, fig. 33) which surrounds the membrane in Salticus contains radiating 

 fibres very similar to those which I have described in the sinus around the margin of the 

 membrana basilaris of the compound eye ; and it is these fibres which contain the nuclei 

 to which Dr. Grenadier believes he has traced retinal fibres. 



At present the origin of the retina of the simple eye cannot be said to have been 

 determined ; I have sought in vain for any reliable indications as to its origin. Dr. 

 Grenadier believes it to arise by a modification of the cells of the hypodermj. His 

 arguments in favour of this origin are very unsatisfactory, and apparently indicate that 

 the vitreous and not the retinal elements arise from this layer. 



A cellular vitreous is always present in the simple eyes of insects : I formerly failed to 

 demonstrate it in the stemmata in Flies, but I have since found that this layer exists, 

 although from its extreme thinness it is not easily seen except in specimens preserved 

 with osmic acid §. 



On this point, at least, I must endorse the views of Von Graber ||, rather than those of 

 Dr. Grenadier. In a very able paper by Prof. E Ray Lankester and Mr. A. G. Bourne ^f, 



* L. c. figs. 12, 95, 96, & 97. f L. c. t L. c. § Phil. Trans. 1. c. 



II Archiv f. mikrosk. Anat. vol. xvii. 1880, p. 58. If Quarterly Journal of Microscopic Science, Jan. 1883. 



Gl* 



