144 PROCEEDINGS OF THE ACADEMY OF [Fob., 



until they come to rest at about one-third of the distance from the 

 cone to the basement membrane, which on account of the tapering of 

 the retinula is at about the centre of the cell, as far as mass of cyto- 

 plasm is concerned. One of the nuclei, however, moves proximally 

 imtil it lies about half-way between the other nuclei and the basement 

 membrane. Where a nucleus is present, the retinula cell is slightly 

 pressed out, encroaching on the outer pigment cells, and the upper 

 nuclei are not all at the same level. The one nucleus which occupies 

 a more proximal level is separated by some distance from any of the 

 others, however, and, owing to the regularity with which it is found, 

 cannot be considered as due merely to a mechanical shifting. In the 

 older stages of development it becomes difficult to count the nuclei 

 of the retinula since they are at different levels, but I have been unable 

 to see anything which would lead me to suspect that this proximal nu- 

 cleus was other than one of the retinular nuclei. Neither is there any 

 indication that the presence of this nucleus is accountable for the pres- 

 ence of nine retinular cells in some ommatidia, for it is found in all 

 ommatidia and the nine-celled condition is comparatively rare. 



The rhabdome differentiation proceeds until it reaches the distal 

 surface of the basement membrane where it ends abruptly. In the 

 pupa stages I am unable to find the nerve fibres which in the adult eye 

 run parallel with the rhabdome and send fine fibrillse into it. It will 

 be noticed, however, that in the pupa the rhabdome is wider and not 

 so definite in outline as it is in the adult eye, and the nerve fibrils are 

 no doubt included in this darker central body which I have identified 

 as the rhabdome. Both rhabdome and nerve fibrilla? are but differen- 

 tiations of the cytoplasm of the retinula cells and their development 

 takes place together. The rhabdome is probably not a uniform struc- 

 ture, but no doubt contains a mass of fibrillse, the endings of the nerve 

 fibres. I am unable to see any such structures, however. 



The development of the retinula consists, then, in the changing of 

 the sense-bud-like spindle of the larval eye into a long column of cells 

 with a clear shaft through the centre, through which light can pass to 

 reach the nerve endings in it. From the previous description it will 

 be evident that the rhabdome is not formed b)^ processes from the cone 

 cells, which are present from the beginning of ommatidial development, 

 but is an intracellular differentiation of the retinula, there being a sharp 

 line of demarcation between the cone cells and rhabdome throughout 

 their development. 



b. The Cone Cells. — The cone cells are four in number, and in the 

 early pupa stage (fig. 3) the cone is spindle-shaped and lies directly 



