146 THE NORTHERN MICROSCOPIST. 
micro spore produces antherida. In Selaginella the rudimentary 
prothallus consists of one cell only. In lycopods we get two pro- 
thalli from one spore. In the Coniferze or Pine family the pollen 
is born on the under surface in pollen sacs, and the grains are two 
to four-celled. The Coniferze form a connecting link between the 
flowering plants and cryptograms. In the ovules of many of the 
higher plants are found what are termed antipodial cells, and which 
are supposed to represent the prothallus of the lower plants, and 
quite recently the pollen grains have been found to undergo divi- 
sion into two or more cells, the largest forming the pollen tube, 
and the others assimilating food for the prolongation of the tube, 
thus forming a rudimentary prothallus. Generally speaking, the 
sexual organs, whether of the lower or higher plants, are but modi- 
fications of the same structure. 
The paper was illustrated by diagrams and microscopic sections 
of the different parts referred to. 
PERMANENT EYE-PICTURES.—After the so-called “visual purple” 
was discovered by Prof. Bell, and it was found possible to profluce 
pictures on the retina, which might be examined after death of the 
animal, the possibility of obtaining such pictures, produced during 
murders, &c., was discussed. In a recent article in the New York 
Medical Journal, Dr. Ayres, who made over a thousand experiments 
in taking ‘“ optograms” on the retina of animals in Prof. Kiihne’s 
laboratory at Heidelberg, comes to a negative conclusion on the 
point. While working in the laboratory, Professor Kiihne proposed 
that he should make a picture of Helmholtz and send it to the 
latter in acknowledgment of the value of his researches on phy- 
siological optics. Dr. Ayres, therefore, got a large negative of 
Helmholtz, and placed it over the eye of an animal, which had 
been dosed with atropine. The animal had been in the dark 
room for hours. The sun was shining brightly, and every precau- 
tion having been taken, the retina was exposed for four minutes. 
There was a dull picture on the cornea, and when the retina was 
examined there was found an image of Helmholtz’s shirt-collar and 
of the end of his nose. The light transmitted through the negative 
was not sufficient to bleach the visual purple. As the purple is 
rapidly regenerated in the living retina, and may have been restored 
in this case as fast as it was bleached, Dr. Ayres cut off the head 
of a rabbit, and waited till all such power in the retina was certainly 
lost. Then he repeated the experiment. The result was a 
little better, but the optogram was by no means distinct enough 
for one to recognise even that it was intended for a picture. 
Dr. Ayres therefore concluded that an optogram could not be so 
obtained. He believes it utterly idle to look for the picture of a 
man’s face, or of the surroundings, on the retina of a person who 
