SCHULTZE, ON THE RETINA. a7 
termed fovea centralis. Some very interesting observations 
are given on the subject of the relation of the diameter of the 
“rods”? and “ cones” to the acuteness of vision, &c. ; and 
the probability is shown that at the point of junction of the 
outer and inner segments of the “ rods ”’ and “‘ cones,’’ which 
differ so much in their refractive properties, and between 
which, as pointed out by Krause, even in the perfectly fresh 
state so sharp a line of demarcation exists, the light passing 
through the retina to the “ rods” suffers reflexion upon the 
end of the imner segment, or upon true percipient nervous 
point, as it may be termed. 
The third section treats of the development of the retina, 
and especially of the ‘‘ rods” and “‘ cones,” and it contains 
many extremely interesting original observations. The 
author’s study seems to have been principally directed to the 
development of the eye in the chick. He shows that the 
pigment-layer of the retina, or the inner layer of the choroid, 
as some deem it, is formed in the outer coat of the primitive 
eye-bulb-sac, and that the outer and at first perfectly even 
surface of the inner coat of the bulb is in close contact with 
the outer. The surface of the inner fold of the primitive bulb- 
sac is formed by, or rather represents, the future membrana 
limitans externa. 'The first indication of the formation of the 
“rods” and “‘ cones ” is visible on the previously perfectly 
even surface of this membrane in the appearance, about the 
tenth day of incubation, upon it of minute hemispherical 
elevations, which are, in fact, the rudiments of those elements 
into which the elevations gradually grow. 
In mammalia the necessary continuous observation is not 
so readily made, but sufficient has been ascertained to show 
that the development of the retina in them proceeds in the 
same way as in the fowl. In fresh embryo calves, in specimens 
from fifteen to twenty-five centimeters in length, the membrana 
limitans externa was in close contact with the pigment-layer, 
and no trace of either “‘ rods” or “ cones” was visible. In 
specimens fifteen to twenty centimeters long, hardened by 
immersion in “ Miiller’s fluid,” or in a weak solution of 
nitric acid, although the nerve-fibre-layer of the retina was 
distinct enough, none of the other layers were as yet differen- 
tiated from the general substance composed of spindle-shaped 
cells haying elongated nuclei and processes passing to the outer 
and inner membrana limitans. 
In embryo sheep, at the time of birth or very nearly so, 
“rods” and “ cones” were present, but not at an earlier 
period. They were, however, shorter, and, above all, much 
more delicate, than in the full-grown animal. 
