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, xviii. 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. 

 ^ Chaps, ii.-ix. Second edition, revised by A. Sedgwick and W. Heape, 

 London : 1883. 



