CHAPTEH VIII. 

 AVES. 



INTRODUCTION. 



THE variations in the character of the embryonic development 

 of the Arauiota are far less important than in the case of the Ichthy- 

 opsida. 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 re- 

 ceived 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 Duck, the 



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

 A. Yellow yolk. 11. White yolk. 



Goose, the Pigeon, the Starling, and a Parrot (Melopsittacus undu- 

 latus) 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 



