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 embryo- 

 logically. The common Fowl has received a disproportionately 

 large share of attention ; although within quite recent times the 



FIG. 85. YOLK ELEMENTS FROM THE EGG OF THE FOWL. 

 A. Yellow yolk. B. White yolk. 



Duck, the Goose, the Pigeon, the Starling, and a Parrot (Melo- 

 psittacus undulatus) have also been studied. The result of these 



B. III. 10 



