MUSEUM OF COMPARATIVE ZOOLOGY. 19 



ous with the groups which pass through the membrane. Each group of 

 large fibres as it passes through the membrane consists of either three 

 or four individual fibres. If the smaller fibres disappear, the groups 

 below the membrane should consist of three or four fibres also ; if, on 

 the other hand, the smaller fibres increase in size, the deeper groups 

 should consist of four or five fibres. By either method of change there 

 would be groups of four fibres, so that it is the groups of three or five 

 fibres which will be decisive. As a matter of fact, the fibres are very 

 commonly in groups of three, and not in groups of five ; consequently I 

 conclude that the smaller fibres dwindle out a short distance below the 

 basement membrane. 



The distal retinulae have not been identified in many Decapods. 

 Carriere ('85, p. 169) has described them in the eye of Astacus. In 

 Penaeus, Patten ('86, p. 634) has observed four cells which belong to 

 the pigmented collar of the retinophora. Two of these, the inner ones, 

 evidently correspond to the distal retinulae of the lobster. They sur- 

 round the cones. The other two, the outer ones, appear to have no 

 homologue in the lobster's eye. 



The Intercellular Spaces of the Retina. 



In the region of the retina which lies between the proximal ends of 

 the cones and the distal border of the deeper band of pigment, the 

 groups of cone-cells and the pairs of distal retinulae are separated by 

 considerable intervening space (Fig. 1, spa,, i cl.). This space is filled 

 with a fluid which contains a very small amount of albuminoid sub- 

 stance. Patten ('86, Plate 31, Fig. 73, x) has figured a similar fluid- 

 filled space in Penaeus. On the application of heat this albuminoid 

 substance in the lobster coagulates and forms larger or smaller vesicular 

 bodies, which vary much in size. They are usually loosely attached to 

 the cone-cells and the fibres of the distal retinulae. They readily take 

 up coloring matter. They have never been observed in fresh retinas 

 when teased in normal salt solution, nor in maceration-preparations. 

 It was probably these bodies which Newton ('73, p. 329, Fig. 15, c') 

 described as the nuclei on the investing membrane. 



In addition to the albuminoid substance which I have described, one 

 occasionally meets with a thin layer of homogeneous material which 

 lies slightly in front of the rounded ends of the proximal retinulae. 

 This forms a dividing membrane which separates the retina into a prox- 

 imal and distal portion. The membrane is of course pierced in many 

 places. There is an opening in it for each pair of distal retinulae, and 



