150 S. WATASB. 



human retina, these two factors are liable to variation in their 

 relative distribution in different Vertebrates. In fact, the 

 variation takes place between the two extremes where the rods 

 alone exist on one hand and where the cones alone constitute 

 the essential part of the retina on the other. Thus, according 

 to Schultze, '' either form of percipient element (rod and cone) 

 may be represented by the other ^^ in the Vertebrate. This 

 range of variability in the distribution of the cones and rods 

 occurs even in a single group of Vertebrates, as in mammalia, 

 showing that the variation in the distribution of the essential 

 factors, even within a tolerably well circumscribed group of 

 animals, is sometimes quite extensive. The group of Arthro- 

 pods is a heterogeneous one, and I see no a priori objection to 

 believing in the existence of a phenomenon analogous to what 

 we find in Vertebrates, viz. that the two percipient elements 

 represented by the central and the peripheral cells in the 

 ommatidiura of Limulus may be differently represented in 

 different Arthropods. 



There is no doubt whatever that the retinula cells are 

 homologous throughout the Arthropods. In fact, in most 

 Arthropods which I have examined no other elements but the 

 retinulse have any connection with the optic nerve-fibres, and 

 they often undergo an enormous development and acquire 

 most complicated structures, as in Homarus or in Pen sens, 

 giving rise to the much discussed " spindle." 



But what has become of the central element which is so 

 conspicuous in the ommatidium of Limulus, if the retiuulse 

 in all Arthropods are homologous ? I believe the central cell 

 is fully functional, judging from its position and from its 

 veritable connection with optic nerve-fibre in Limulus. 

 What in other Arthropods strongly reminds one of this cell is 

 the "hyaline celP^ at the bottom of the ommatidial pit in 

 Serolis and, according to Beddard, also in the Cymo- 

 thoidse. One important difference, however, exists between 

 the " hyaline cell '' of the Isopods and the central cell in the 

 ommatidium of Limulus, viz. that, while in the latter the 

 cell is connected with the optic nerve, the " hyaline cell " in 



