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 miht) 
beneath it, which he terms a rhabdome (Rm). 
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 larve and pupz 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 trachez are seen between the rhabdomes with 
branching nucleated cells on their walls (Pl. XXXVIII., Fig. 1). 
