16 BULLETIN OF THE 



Secondly, in isolation-preparations the thickenings always remain at- 

 tached to the retinulae ; the cones, on the other hand, are covered with 

 membranes of uniform thickness. The thickening in the membrane 

 is not characteristic of the whole length of the retinula, but is pecu- 

 liar to the region corresponding in level to the distal end of the cone 



(Fig- !)• 



The foregoing description applies to the structure of the distal 



retinitis as seen in the plane of Figure 4. This plane passes through 

 the outer ends of the cones. In other regions the retinulae present 

 somewhat different conditions. The relation of the retinula? to the 

 hypodermal squares is shown in a section (Fig. 3) which is slightly 

 more superficial than that just described. The two retinulae which 

 were located at the two angles of the cones here occupy the corre- 

 sponding angles of the hypodermal squares. They do not, however, 

 entirely cover the four lateral faces of the square, as they did those of 

 the cone, but from the angles at which they are located they extend 

 over half of each of the adjoining faces. It follows from this that 

 together they flank only one half of the lateral exposure of the square. 

 The blades are now no longer wedge-shaped in transverse section, nor 

 do they overlap neighboring blades, but each one stretches completely 

 across the space in which it lies. Of the lateral surface of the square, 

 that half which is not sheathed by its own pair of retinulae is covered 

 by the arms of four adjacent retinulae. Consequently, six retinulae in 

 all touch each hypodermal square. Two of these belong to the omma- 

 tidium which is represented by the square ; four belong to adjoining 

 ommatidia. The relation of these will be readily seen by referring to 

 Figure 3. 



In passing from the plane in which each cone is surrounded by its 

 own pair of retinulae to the one in which the corresponding hypodermal 

 square is surrounded by six retinulae, the blades of the two retinulae 

 proper to the cone undergo a gradual narrowing ; so that, instead of 

 each blade covering the whole of one face of the cone, it covers less and 

 less, and eventually sheathes only one half of the corresponding face of 

 the hypodermal square. As the blades of the retinulae become narrower, 

 they expose the surface of the cone, but this is still kept covered by the 

 retinulae of adjoining ommatidia. In any ommatidium there are four 

 blades which become narrow, consequently there are four regions in 

 which adjacent retinulae touch the cones; and as there is a separate 

 retinula for each region, it follows that four additional retinulae here 

 come in contact with the cone. 



