NOTES AND MEMORANDA. 
225 
doubt, who will examine a common calcareous sponge, or who looks at 
Barrois’ valuable memoir on the subject, that F. E. Schulze was — as 
he himself has admitted — so far misled in his account of the develop- 
ment of Sycandra rceplianus as to transpose two very important stages 
of the development. In fact, the concavo-convex stage of the embryo 
sponge, with one ^et of cells (endodermic) tucked into the narrower, 
clearer, longer, ciliate cells, actually precedes that in which the same 
cells form respectively a hemisphere of clear ciliate cells and a hemi- 
sphere of large swollen cells, not tucked into the former at all, but so 
arranged that a small central cavity is closed in by the two groups. 
How we pass from this stage to the young sponge, or even to the two- 
cell-layered sac, is still a complete mystery. One thing, however, is 
obvious. Haeckel could hardly have been led to the generalization 
known as the gastraea theory, which, on the whole, is a truthful and 
productive generalization, by erroneous observation. We must, there- 
fore, respect his positive statements of fact. 
The Eyes of Insects . — In a paper by Mr. B. T. Lowne, on “ The 
Modifications of the Simple and Compound Eyes of Insects,” read 
before the Koyal Society, the author states that in his opinion the 
extent and curvature of the cornea and the size and curvature of the 
facets afford the most important indications as to the manner in 
which vision is accomplished. In the true compound eye, he thinks 
the structure indicates that J. Muller’s theory of vision is the most 
probable ; this is also Dr. Grenacher’s view, and it is supported by the 
curvature of the cornea and the size of the corneal facets in different 
insects, as well as in different parts of the same eye. 
The semi-compound eye introduces no new difficulty in this theory. 
In order to determine the effect of the long, fine, highly refractive 
threads of the eyes of insects upon the light, he made some experi- 
ments on the transmission of light through fine threads of glass. 
He took a capillary tube of glass 5^^ of an inch in thickness, 
about of an inch in diameter, and an inch in length, placed it 
upright in a small trough under the microscope and examined it with 
an inch objective. He found that no light passed through the lumen of 
the tube, but that the section of the wall of the tube appeared brightly 
illuminated. He next placed a few fine glass threads, drawn from a 
glass rod, in the interior of the tube ; these were as nearly as possible 
the same length as the tube and measured y^Vo diameter. 
The upper end of each of these rods appeared as a brightly illuminated 
disk in the dark field ; when the focus of the microscope was altered, 
the disk was enlarged, showing that the rays left the rod in a divergent 
direction ; in some cases when the ends of the rods lay beyond the 
focus of the microscope, the disks of light exhibited grey rings, the 
result of interference. 
When the lower ends of these rods were lenticular, or fused into 
a drop, or drawn into a core, the phenomena were the same, and in all 
cases the action of an oblique pencil, even when the obliquity was 
very slight, was feeble as compared with that of a pencil having the 
direction of the axis of the rod. 
These results are such as would be predicted on theory ; all the 
