[60] Zeit. f. wiss. Zool., Bd. XXV. sup. 1875.
[61] Archivio per la Zoologia, l'Anatomia e la Fisiologia, Vol. 1. Genoa, 1861. Conf. also Von Siebold, “Ueber die geschlechtliche Entwicklung d. Urodelen-Larven.” Zeit. f. wiss. Zool., Bd. XXVIII., 1877.
CHAPTER VIII.
AVES.
Introduction.
The variations in the character of the embryonic development of the Amniota are far less important than in the case of the Ichthyopsida. There are, it is true, some very special features in the early developmental history of the Mammalia, but apart from these there is such a striking uniformity in the embryos of all the groups that it would, in many cases, be difficult to assign a young embryo to its proper class.
Amongst the Sauropsida the Aves have for obvious reasons received a far fuller share of attention than any other group; and an account of their embryology forms a suitable introduction to this part of our subject. For the convenience of the student many parts of their developmental history will be dealt with at greater length than in the case of the previous groups.
The development of the Aves.
Comparatively few types of Birds have been studied embryologically. The common Fowl has received a disproportionately large share of attention; although within quite recent times the Duck, the Goose, the Pigeon, the Starling, and a Parrot (Melopsittacus undulatus) have also been studied. The result of these investigations has been to shew that the variations in the early development of different Birds are comparatively unimportant. In the sequel the common Fowl will be employed as type, attention being called when necessary to the development of the other forms.