RETINA OE THE BLOWFLY. 
413 
In transverse sections, when unaltered by the process of im- 
bedding, they are circular or hexagonal rings, with, a large 
central cavity; they touch each other at their periphery, and the 
tracheal vessels appear as thick-walled but very small tubes. 
Each great rod is seen in such sections to be lined by a thin 
cuticular layer, which dips down between the sheathing cells ; 
it is the folds of this membrane which appear as bright highly 
refracting points under unfavorable conditions of illumination. 
With direct central light, thin sections, with oil or water 
immersion-lenses, no longer present these appearances ; there 
is no bundle of axial rods in such preparations when properly 
examined, only a thin cuticular lining. 
Further evidence in favour of my views is, I believe, shortly 
forthcoming from the pen of an independent observer. Prof. 
Plateau informs me that last year, at Cologne, Dr. Exner showed 
the single image formed by the compound eye — the image in 
the plane of my basilar membrane formed by the uninjured eye, 
i. e. by my dioptron. I wait anxiously for the spring, as with 
fresh insects at command I have little doubt the demonstration 
of an erect picture in this region is perfectly easy. 
The Development of the Compound Dye. 
The development of the compound eye was described by Weis- 
mann in 1864 *. I have gone through a most laborious research, 
and in the main points my observations agree with those of the 
great Grerman investigator. W eismann says it has long been 
known that the eye in insects is developed from two perfectly 
distinct parts — one from the nerve-centres of the larva, the other 
from the optic disc (“ Augenscheibe,” l. c. p. 194). 
If we follow the development of the optic disc, we find it at first 
as a thin cellular expansion enveloping the anterior part of the 
hemisphere (or supra-oesophageal ganglion). It consists of cells 
(the optogenic cells of Yiallanes) which are larger than those 
of the other discs ; they measure 15 g in diameter at an early 
period of the pupa state and have large clear nuclei. During 
the formation of the head, the eye- disc separates considerably 
* “ Die Entwicklung der Dipteren,” Leipzig, 1864. Reprinted from Roll. 
Zeitsch. f. w. Zool. 
31 * 
