18 BULLETIN OF THE 



iu Figure 18, /•<«,'. dst. Beyond this I have not been able to trace 

 them with certainty. The groups are no longer observable, and it is 

 probable that the fibres have separated. I know that in this region the 

 other cells suffer a very considerable rearrangement ; and such being 

 the case, it would be a very difficult matter to identify single fibres, 

 especially fibres as small as these are. I have not found any satisfactory 

 method of staining the fibres so as to distinguish them, as in the case of 

 the fibrous ends of the cone-cells. I can therefore claim to have traced 

 the fibres only to within about 20 p. from the basement membrane. 



As I have already mentioned, the distal face of the basement mem- 

 brane has cross-shaped thickenings on it. In the angles which the arms 

 of the cross make with each other, the basement membrane is perforated. 

 There are consequently around each cross four openings through the 

 membrane. Each opening, however, lies between two crosses, so that in 

 reality only one half of each opening belongs to a given cross, or, if one 

 counts whole openings only, half of the four openings, i. e. two openings, 

 belong to each cross. The crosses correspond in number and position 

 to ommatidia, hence there are also two openings for each ommatidium. 

 In each opening, beside three or four large fibres which will be described 

 later, one finds a single small fibre (Fig. 21, rtJi'. dst.). That this fibre 

 represents the continuation of the fibrous end of a distal retinula seems 

 probable, for two reasons. First, the diameters of this fibre and of the 

 fibrous part of the distal retinulse are so nearly the same as to be undis- 

 tinguishable. Secondly, the number of fibres which pass through the 

 basement membrane, two for each ommatidium, agrees with the number 

 of distal retinulae in each ommatidium. I therefore believe that the 

 small fibres which are seen in the openings through the basement mem- 

 brane are the proximal continuations of the fibres of the distal retinulae. 

 If this explanation be true, then it is only natural to expect that, as a 

 pair of fibres approaches the basement, the individual fibres should 

 separate, one passing through each opening. As I have already ex- 

 plained, the fibres, while separated, could be identified only with great 

 difficulty. 



If the fibres pass through the basement membrane, as I believe they 

 do, they terminate only a short distance below it. For at about 15 /a be- 

 low the membrane all of the fibres are of nearly the same size, i. e. some- 

 what larger than the large fibres which pass through the membrane 

 (Fig. 22). In this region, then, the smaller fibres have either increased 

 to the size of the larger ones, or diminished till no trace of them is left. 

 The fibres here are in groups, however, and these are directly continu- 



