MORPHOLOGY OF COMPOUND EYES OF ARTHROPODS. 151 



the former lias no connection with the central nervous system 

 whatever. Hence the " hyaline cell " cannot be sensory, even 

 if it be homologous with the central cell of Limulus, which 

 it resembles in its general appearance and in its position. 

 The number of ^^ hyaline cells ^^ in Scrolls is always two, 

 while its supposed homologue in Limulus is^ as a general 

 rule, only one. This fact does not offer any objection to my 

 view of their homology when we bear in mind that other 

 elements in different ommatidia, as vitrellse and retiuulse, 

 show a wide range of variation so far as their numbers are 

 concerned, and yet they can be considered as perfectly 

 homologous. 



A further embryological and comparative knowledge in 

 regard to the " hyaline cell " in Isopods is necessary for the 

 determination of its exact homology. Meanwhile I would 

 observe that if the central and the peripheral cells which we 

 see in the ommatidium of Limulus may be taken as the two 

 essential factors of the sensory element of the typical Arthropod 

 retina, the case of Serolis may be taken as a loss of balance 

 in the relative development of these two factors, the central 

 cells having lost their sensory function and remaining as a sort 

 of supporting mechanism. We can imagine this change in the 

 function of the central cell as carried still further, and with 

 the excessive development of the peripheral elements, the 

 retinulse, the central element may finally have disap- 

 peared. 



All this is, however, a mere suggestion, and my interpreta- 

 tion of the nature of the Arthropod ommatidium in general 

 does not lose its force even if this section of my views in regard 

 to the fate and homology of the central cell or cells be proved 

 untenable. It is quite possible that the ommatidia in which 

 there is no element corresponding to the central cell of 

 Limulus may have originated without it from the beginning. 

 It seems, however, more natural to suppose that such an 

 ommatidium had it originally and lost it later, observing that 

 the simplest form of ommatidium possesses it in its fully 

 functional, sensory form. 



