EMBRYOLOGY 199 
The production of eggs does not necessarily depend upon previous 
fertilization by the male, as shewn by numerous instances of birds 
which have laid eggs although they had been kept in absolute 
celibacy. 
A most important, but still unexplained, allegation is that 
eggs, containing hybrids, are not exactly like the eggs of the race 
or species of the female, but more or less resemble also the eggs of 
the race or species to which the fertilizing male belongs. Instances 
of such mongrel eggs are mentioned by Nathusius (Zeitschrift f. 
wissensch. Zoologie, xvii. p. 229); and other well-authenticated 
instances would form valuable contributions to any of our scientific 
periodicals. 
During the descent of the fecundated egg along the oviduct, 
where it is exposed to the temperature of the bird (about 
40° C.=104°F.) the germinal disk has already undergone important 
changes; repeated divisions, or segmentation having transformed 
the disk into a large number of small rounded masses of protoplasm, 
or cells. Between this segmented disk or “blastoderm” and the bed 
of white yolk on which it rests, a space containing fluid makes its 
appearance. The central, greater part of the disk, so far as it 
overlies the fluid-containing space, is transparent and distinguished 
as the area pellucida from the area opaca or opaque rim of the disk, 
which rests immediately upon the white yolk within the vitelline 
membrane. 
When the egg is laid and becomes cold these changes all but 
entirely cease, and the blastoderm remains inactive until, under 
the influence of the higher temperature of incubation, the vital 
activities of the germ are again brought into play, ushering in 
the series of events by means of which the development of the 
individual bird is accomplished. No better description of them, as 
they occur in the Common Fowl, can be found than that given in 
Foster and Balfour’s Elements of Embryology, of which the following 
is a condensed account, and to that admirable book! the student 
may be referred for further detailed information. 
It is convenient to begin with a preliminary general sketch of the 
development of the embryo. The embryo itself is formed entirely 
in the area pellucida; the structures to which the area opaca gives 
from deception, but the ingenious and simple experiment made by Mrs. A. Ernst 
(cf. Zoologischer Anzeiger, viii. 1885, p. 718) could easily be repeated. The birds 
were kept upon moist sand and charcoal, and when the cackling of a hen indi- 
cated her safe delivery, the egg was inspected and invariably found to be black- 
ened at the blunt end. Unless it be assumed for argument’s sake that the egg 
while dropping had time to turn round with its heavier pole downwards, this 
test seems to be conclusive, but of course it does not exclude wrong presentations, 
1 Chaps. ii.-ix. Second edition, revised by A. Sedgwick and W. Heape, 
London: 1883. 
