THE DEVELOPMENT OF THE COMPOUND EYE. 549 



terms a crystalline cone (Kk in his figure), is so evidently 

 a rhabdome, whilst he actually figures a retina (Sensu mihi) 

 beneath it, which he terms a rhabdome (Rin). 



The Origin of the Rhabdome from the mesoblast was certainly 

 unexpected by me until very recently, but this origin accounts 

 for the very remarkable similarity which exists between the 

 rhabdomes of insects and the tracheal vessels, and it further 

 accounts for its intrusion into the epithelial layers as well as 

 its intimate relation with the retinal end organs of Hyperia, of 

 the- simple eyes of Caterpillars, and in the compound eyes of the 

 larval form of the Ametabola as well as of the Crustacea 

 generally, for although in the higher Insects it is always easy 

 to separate the retina from the rhabdomes, in many Arthropods 

 the connection is undoubtedly very intimate, and it is possible 

 that in some no retina exists, the eyes being in that case 

 functionless, as they are undoubtedly in the Blow-fly nymph, 

 and in the larvae and pupae of most Metabola. 



In this stage of the Blow-fly nymph I have been quite unable 

 to discover any retinal end organs, but if the connective cell 

 uniting the neural disc with the rudimentary rhabdome were 

 replaced by a nerve fibre ending in a single retinal end organ, 

 or by a small bundle of nerve fibres terminating in a group of 

 end organs, the condition of the eye would be similar to that 

 seen in the simple eyes of Caterpillars, in the ommatea of the 

 larval Gnat, and in such Crustacea as Hyperia. The absence 

 of nerves and end organs in the rudimentary ommatea of the 

 Blow-fly nymph is quite explicable as the result of their total 

 want of functional activity. 



Fourth Stage of Development (tenth or eleventh day of the 

 pupa). In this stage the rhabdomes are greatly increased in 

 length, so that the epidermal disc has attained a thickness of 

 125 mm. The number of cells is apparently the same as at 

 the earlier stage, but as these have not become greatly enlarged 

 they are more widely separated. The principal change which 

 has occurred is that the ovoid body has become cylindrical, 

 and numerous tracheae are seen between the rhabdomes with 

 branching nucleated cells on their walls (PL XXXVIII., Fig. i). 



