42 THE OVUM AND 
by a very delicate pellicle, the albumen-membrane, 6, also that 
the transparent substance itself (albumen) is sufficiently fluid to 
permit of such a degree of displacement of the yelk as to allow 
of its coming into contact even with the albumen-membrane. 
Although I have never yet succeeded in observing this pellicle, 
and though in my researches the transparent membrane, on 
the bursting of the yelk, always tore with smooth edges like a 
solid substance, yet the observations of the respected discoverer 
are too precise to admit of a doubt upon it. It is also sup- 
ported by the analogy of most of the ova of other classes of 
animals, in which chorion and vitellme membrane may gene- 
rally be distinguished, notwithstanding that they sometimes lie 
close upon each other. The albumen-membrane has probably 
the signification of a cell-membrane, in which case the albumen 
will be the cell-contents, and the yelk a young cell. Accord- 
ing to Wharton Jones, the transparent areola (zona pellucida) 
of the ovum, or the albuminous layer in the fecundated 
ovum of mammalia, becomes considerably expanded in the 
tubes, a fact which would be readily explained by the inherent 
energy of the albumen-membrane when regarded as a cell. 
In such case, however, the mode of formation of the albumen 
would be very different from the corresponding process in the 
bird’s egg, where, according to Purkinje, it is secreted by the 
oviduct, and a membrane (chorion) is formed around it sub- 
sequently, which cannot therefore have the signification of a 
cell-membrane, and is moreover not simple in structure, but 
composed of fibres. Meanwhile an investigation might be 
made, as to whether the albumen in the egg may not also be 
first surrounded and formed by an equally thin pellicle, 
around which a secondary external membrane may subsequently 
be produced. According to Purkinje, however, this is not 
the case, and I could not discover any such pellicle upon the 
inner surface of the shell-membrane of the excluded egg. I 
have not made any inquiry as to whether the chorion of fishes 
is a cell-membrane or not. It is covered internally with a 
very beautiful epithelium, which is made up of more or less 
flat hexagonal cells, each of which has its nucleus. 
Within the transparent areola, or, according to Krause, the 
albuminous layer, lies the vesicle of Baer, or the yelk ; which, 
from Krause’s statement, is enclosed by a peculiar structureless 
