Cuap. I. ORIGIN OF THE EGG. 455 
tenacity, holds together for a while longer, but finally disintegrates and discloses 
its mode of origin in the immense number of minute, faint granules, identical in 
At times 
a granular structure, giving to the wall the appearance of a cellular membrane 
appearance with the extruded ones, which are dancing about in zigzag. 
on a very minute scale," may be detected, even before it has lost its consistency. 
As the egg grows larger, the dark outline decreases in thickness, and the 
contents become less oily and more transparent, till at last, when it is about 
zoo Of an inch in diameter, a well defined wall discloses itself under the guise 
of a thin pellicle (Pl. 8, fig. 1, f, 9). 
The study of a series of eggs,” such as have just been presented,—in which at 
first no wall is visible, then faint indications of a superficial change appear, in which 
a gradual differentiation of the parietal from the more internal substance ensues, and 
the finally well established separation of the two is unmistakable, the latter, the inter- 
nal, inclosed by the former, which presents itself as a sharply pronounced, extremely 
tenuous envelope,—leaves no doubt that the egg-cell wall has an origin totally exter- 
Whether this 
wall has arisen by a gradual change in the density of the superficial particles, or by 
nal to all that which is inclosed in it at the time it becomes visible. 
original deposition in its present form, it is impossible to determine; but this much 
is demonstrated, that at least a small portion of the egg elements exists before 
its wall has become established, and that this wall, far from beimg the nidus in 
which its contents are developed, is more probably the offsprmg of what it incloses. 
It would be more proper, perhaps, and nearer the true nature of the operation, 
to say that the yolk membrane arises synchronically with the concretion of the 
original yolk particles, as a denser exterior stratum, which, subsequently becoming 
1 Indeed, it is no exaggeration to say, that such is 
essentially a cellular membrane ; for as cells originally 
are what we have designated as granules, and cells 
unite to compose a membrane, why may not gran- 
ules, cells, combine to make a wall around a certain 
substance ? In some respects it is only a matter of 
size, after all; put on the higher powers of the mi- 
croscope, and the granules may appear so large that 
they would be called cells by every observer; and 
what are minute cells under three hundred diame- 
ters, are, to the eye, mere granules with thirty or 
forty diameters. 
2 Tn order to preserve the natural relations which 
exist between the many isolated figures drawn to illus- 
trate the structure of the eggs of different stages of 
development, it has been necessary to adopt a very 
peculiar mode of numbering and lettering the figures. 
Yet, as it is not possible to describe at once all the 
different features which these figures are intended to 
bring before the eye, it may facilitate the understand- 
ing of the following pages, if the reader will first 
make himself familiar with the arrangement of the 
Plates 8,9, and 9a, by studying the explanation which 
accompanies them. The student already familiar 
with Embryology may also read with advantage, Sec- 
tion 5 of this Chapter, before any other. 
Tn the quotations, the reader ought to mark care- 
fully the difference between the letters following the 
figures without comma, which indicate the whole 
figure, and those following a comma and are referred 
to in italics, which designate the individual illustra- 
tions belonging to the same objects. 
