410 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



In many perfect insects, and in some stages of development in others which in the 

 perfect form bave a continuous retina, conditions exist which are intermediate. For 

 instance, the bacillary layer may be continuous, but the optic nerve split into numerous 

 separate bundles ; or the segregate retina may consist of fasciculi so closely united that 

 it is difficult to decide to which form the modification should be assigned. These varia- 

 tions are of developmental significance, as will be shown hereafter. 



The fibres of the optic nerve are medullated, although in most preparations all traces 

 of the medullary sheath are lost. It is not, however, difficult to make out the sheath in 

 recent specimens fixed with osmic acid. I now consider the varicose appearance of the 

 axis cylinders which I formerly described to be due to post-mortem changes * . 



Tbe arrangement of the fibres in separate fasciculi in some insects has aheady been 

 alluded to. When they form a single nerve-trunk most sections exhibit the complete 

 decussation of the nerve-fibres. In some planes, however, tbe only indication of such an 

 arrangement is the cut ends of many of the nerve-fibres. Such variations are undoubtedly 

 due to tbe plane of the section. 



When the retina is segregate, the nervous bundles, which unite the retina with the 

 ganglion, do not decussate ; but in this case a deeper layer of decussating fibres can 

 usually be seen in the substance of the ganglion itself (fig. 44). 



Supporting fibres connect the inner surface of the retina with the outer surface of the 

 ganglion, when the nerve consists of a single trunk (fig. 56, cl). 



The retinal ganglion is spread over the surface of the optic lobe (oj).) of the cephalic 

 ganglion (figs. 48 & 49). 



It usually exhibits at least four layers, although the number is sometimes greater. 



In the Ply (fig. 56) the outer layer (f) consists of small round nuclei imbedded in 

 a finely granular matrix. Tbe matrix is permeated by fibres connected with a layer 

 of stellate cells (g), which lie in the central zone of the next layer. 



The second layer, beside containing the stellate corpuscles just alluded to, is chiefly 

 made up of large fusiform cells (cl 1 ), which are connected at both their extremities with 

 fine fibres. The third layer of the ganglion consists of a number of very fine fibres 

 (//■), which run parallel to the surface of the ganglion. The supporting fibres of the first 

 and second layers spread out into foot-like disks on the surface of the fibrous layer (i). 

 The fourth layer (cl 2 ) resembles the second, except that I have not detected any stellate 

 corpuscles in connexion with its supporting fibres. 



The retinal ganglion is connected with the deeper portions of the optic lobes by fibres 

 (no 2 ), between which a large number of tracheal tubes lie. These supply the fine tracheal 

 vessels of the ganglion itself. 



In the ganglion of the retina in Agrion (fig. 42) there are two extra layers within the 

 inner cellular layer — a second layer of fibres parallel with the surface of the ganglion (f 2 ), 

 and a second nuclear layer (ii 2 ) ; and the ganglion is connected with the deeper structures 

 of the nervous system by a second layer of decussating fibres (x). 



In Tipula and the Gnats the structure of the ganglion is not so complex. In these 



* Phil. Trans. I. c. p. 585. 



