408 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



outer segments iu many larval forms, in the imago of Tipula and in most Noctuid Moths. 

 In these cases a long fine axis-cylinder passes from the outer to the inner segment 

 (figs. 46, 55, 57, & 67). 



In the Noctuids a tracheal network is interposed between the inner and outer segments 

 of the hacilla, permeated by the fibres which connect them with each other. The 

 tracheae form a kind of tapetum behind the outer segments. 



A similar separation of the inner and outer segments of the bacilla occurs in the 

 simple eyes of an Arachnid (Phalcmgium), which has been figured by Dr. Grenadier *. 



It will be observed that the outer segments of the bacilla, which correspond in cha- 

 racter with the outer segments of the vertebrate rods, are turned towards and not away 

 from the refractive media. In this they conform to the usual condition in the inverte- 

 brate eye. I regard this as of developmental rather than of functional importance. 



The outer ends of the bacilla are separated from the basilar membrane by a layer of 

 fine branching endothelial cells. These are frequently pigmented, and send pigmented 

 fringes inwards, which closely embrace the outer segments of the bacilla. The pigment 

 ^is not deposited between the bacilla and the membrane, but only around the bacilla. I 

 regard this layer as the analogue of the retinal pigment. It is always black. The 

 pigment is entirely wanting in the Diurnal Flies, but is very abundant in the Lepidoptera, 

 Hymeuoptera, and Neuroptera. 



The neuroglia of the reti na consists of fine fibres (fig. 56), connected with the basilar 

 membrane and with two distinct layers of small stellate cells — an outer layer between 

 the outer and inner segments of the bacilla (a), and an inner layer at the inner extre- 

 mities of the inner segments (»r). In many specimens a number of small granules are 

 seen crowded together between the outer segments of the bacilla (fig. 59). I am not 

 certain of their nature, but it has occurred to me that they are possibly the broken outer 

 ends of the bacilla themselves. 



Sometimes large soft granular cells are seen in the same position (fig. 58). I think 

 these are an indication of immaturity. 



None of my predecessors in this investigation have described the bacilla. It may, 

 therefore, be fairly asked, If these structures exist, how is it that they have been so 

 frequently overlooked ? 



I think the answer to this question is found in the fact that it is only in very thin 

 sections, such as they had not the means of preparing, that the bacilla can be recognized. 

 In all ordinary sections a thick band of pigment between the membrana basilaris and the 

 optic nerve is all that can be made out. When I wrote my former paper I had only 

 seen the inner segments of the bacilla in the Flies, and I described them as a fascelloid 

 layer f . I did not then understand their significance, and I had been working at the 

 compound eye for over three years before I prepared a section which enabled me to 

 recognize these structures as the terminal organs of the optic nerve. 



Again, although the structure of the great rods has been very carefully investigated, 

 that of the neuron has had very little attention paid to it. Even Dr. Grenacher, in his 



* L. v. fig. 15. t Li. c. Phil. Trans. 



