586 SUMMARY OF CURRENT RESEARCHES RELATING TO 
distribution of marine animals is chiefly regulated by temperature. We 
may suppose that abyssal forms will become extinct in the Mediterranean 
and that their place will be taken by forms occupying more shallow 
waters and better adapted to higher temperatures. 
Arthropoda. 
Signification of Vitelline Cells in Tracheata.* — Mr. W. Schimke- 
witsch points out that in Amphibia and several Tracheata the cavity of 
the mesenteron is surrounded by elements of two kinds — the cells of one 
side are deprived of vitellus, while those of the other are true vitelline 
cells. These latter are differentiated in very early stages, sometimes 
during the segmentation of the egg ; they may take part in the formation 
of the epithelium of the mesenteron. It is very probable that in those 
Tracheata in which the rudiment of the internal lamella, formed by 
invagination, is destined entirely for the formation of the mesoderm, that 
the epithelial layer of the mesenteron is developed exclusively at the 
expense of the vitelline cells. These cells in Amphibians and Tracheates 
are elements which long preserve tbeir embryonic character, but from 
the morphological point of view they belong to the endoderm. It remains 
to be seen whether they are comparable to the vitelline nuclei of other 
Vertebrates. 
a. Insecta. 
The Retinal Image of the Insect Eye.j — Prof. Exner believes that 
he has been able to settle the controversy as to whether creatures pro- 
vided with facetted eyes see by one erect image or by many inverted 
ones, in favour of the first hypothesis. In the case of the glow-worm 
( Lamjoyris sjplendidula ), he has succeeded in demonstrating this erect 
image, and has shown that the dioptric apparatus of the eye is of such a 
kind that the distance of the image from the refracting media increases 
with the distance of the object from the eye. The two focal points lie 
on the same side of the refracting media, and by transmission of the 
rays in the opposite direction, a virtual inverted image is produced, 
which has the same position with regard to the refracting media as 
the erect image. The eye has no optic axis in the ordinary sense of the 
word, and the retinal image lies on a spherical surface parallel to 
the outer curvature of the eye. 
It was in 1826 that J. Muller proposed his theory of the erect 
retinal image in the insect eye. According to this theory each element 
or facet consists of a transparent tube, coated with black pigment. 
These tubes are arranged in radial position on a hemisphere. Thus for 
each tube only rays incident in the direction of the radius can reach the 
retina at the extremity, while rays at any other incidence are absorbed 
by the pigment. An erect image is accordingly formed on the hemi- 
spherical convex retina at the base of the tubes. This theory was 
supposed to have been refuted by the observation made by Gruel and 
Gottsche, that, under certain conditions, an inverted image corresponding 
to each facet of the eye of a fly could be seen under the Microscope. 
* Zool. Anzeig., xiii. (1890) pp. 399-402. 
f SB. K.K. Akad. Wiss. Wien, xcviii. (1889) pp. 13-65, 143-51 (3 pis. and 
7 woodcuts). 
