MOEPHOLOGY OF COMPOUND EYES OF ARTHEOPODS. 149 



important factor in the omraatidium of Limulus, nor can we 

 doubt the existence of a fundamental homology between the 

 retiuulse of Limulus and those of all the other Arthropods 

 which I have examined. With the exception of a few pro- 

 blematical bodies, such as the "hyaline cells ^■' of Scrolls, 

 there are no structures in the ommatidia of most Arthropods 

 which correspond to the central ganglion-cell of Limulus, in 

 spite of the existence of a fundamental homology in the other 

 elements of the ommatidium. 



"What has become of the central ganglionic element of the 

 ommatidium ? Was it lost in the course of the phylogenetic 

 history of a more complex ommatidium ? Or is it reasonable 

 to suppose that some ommatidia came into existence without it 

 from the beginning? Or, if it were lost at all, is there any 

 evidence which makes this supposition probable ? 



The colourless ganglionic cell and the pigmented rod- 

 bearing cells which surround the former I consider as the two 

 primitive morphological factors in the unit of the sensory part 

 of the Arthropod retina, somewhat in the same way as the 

 circle of rods with a cone in the centre are the two essential 

 factors in the neuro-epithelial layer of the human retina. 

 In the absence of enough comparative data in Arthropods at 

 present we have to dwell largely on the analogy suggested in 

 the other groups of animals. Whatever be the views as to the 

 fundamental homology of the ommatidium of Limulus to a 

 structural unit of the sensory part of the human retina, a 

 superficial resemblance of the one to the other is certainly very 

 strong. The structural resemblance is paralleled by a physio- 

 logical one. The place where the light acts in the visual end- 

 organ of Arthropods and of man may alike be considered as 

 consisting of a number of definite groups of cells, each group 

 being a morphological and a physiological unit; or, in other 

 words, the sensory part of the retinae in both cases consists of 

 a mosaic of several sensitive spots. The image formed on such 

 a surface is therefore a mosaic one, whether in an Arthropod 

 or in a Vertebrate. 



Fundamental as this arrangement appears to be in the 



