550 THE SENSES AND SENSORY ORGANS. 
The fact that each rhabdome has only a single nucleus on 
its surface which projects from it, is very manifest at this and 
in all subsequent stages in the nymph; such rhabdomes are 
figured by Weismann [2 (Fig. 55, F.)] with only a single nucleus. 
The epithelial elements at this stage are four sub-corneal 
nuclei, the five iris cells and five cells at the inner extremity of 
each rhabdome. 
In Immature Diptera, Eristalis and Musca even some hours 
after their escape from the pupa, the rhabdomes are seen to be 
connected in some specimens at their inner extremities with 
large stellate cells (Pl. XXXIX.) which lie in great numbers 
between the outer surface of the retina and the inner surface 
of the dioptron in the pupa. I formerly figured this connection 
[219, Figs. 9, 12, and 13],and mistook them for ganglion cells; 
these cells gradually disappear as the retina comes up to the 
basilar membrane. I have recently examined a number of 
specimens prepared by my friend Brigade-Surgeon Scriven, 
in which the connection of these cells with the rhabdomes is 
very distinctly seen in an immature imago, although the retina 
is close to the inner end of the rhabdomes, and the cells in 
question are flattened. These cells surround the trachea of 
the sub-dioptric space, but are distinctly separated from 
the retinal end organs by the flattened pre-retinal cells 
(Pl. XXXIX. $7). 
The Morphology of the Rhabdome.—It appears to me that the 
rhabdome must be regarded as a mesoblastic, or parablastic 
structure, developed from the same kind of cells as the tracheal 
vessels. It is worthy of remark that the formation of a 
cuticular sac within a single cell only occurs in the trachez and 
the rhabdomes ; and when the tracheal intima is first developed 
within the cells from which the smaller trachez originate, this 
sac is filled with fluid, which is only subsequently replaced by air. 
That the parablastic elements of the trachee penetrate the 
dioptron is undoubted, and in many of my preparations the 
gelatinous ovoid body from which the rhabdome is developed 
is undoubtedly connected with the branching cells from which 
the trachez originate. In its mesoblastic origin the rhabdome 
