222 THE EVOLUTION OF IVIAN. 



of the small Osseous-fish which we have been examininsf. 

 and of the huge eggs of the Primitive Fishes (Selachii), of 

 Reptiles^ and of Birds. 



The egg of the Bird is specially important to us, for 

 most of the important researches into the evolution of 

 Vertebrates have been founded on study of incubated hen's 

 eggs. It is much harder to procure and to examine mam- 

 malian eggs ; for which very practical and incidental reason 

 the latter has been more rarely accurately studied. On 

 the other hand, hen's eggs can always be obtained in 

 any quantity, and artificial hatching enables us accurately 

 to follow every stage in the changes undergone by the 

 embryo in the course of its evolution. As we have seen, 

 the chief difference which distinojuishes the e^s of the 

 Bird from the minute egg of the Mammal is the very con- 

 siderable size of the former, which is due to the accumula- 

 tion of a very large mass of fatty nutritive yelk. This is 

 the yellow mass which, daily consumed under the name of 

 yelk of egg, is collected within the original yelk or proto- 

 plasm of the egg-cell. In order to obtain a correct con- 

 ception of the Bird's egg, the nature of which has very 

 frequently been misrepresented, we must search for it in 

 its earliest condition, and follow its evolution from its 

 beginning in the ovary. In this stage, we find that the 

 original egg is a very small, naked, and simple cell with 

 a nucleus, and that it differs neither in size or shape 

 from the original egg-cell of Mammalia and other animals. 

 (Of Fig. 10 E, p. 134.) As in all Skulled-animals {Craniota) 

 the original egg-ceU or primitive egg (jprotovum) is com- 

 pletely covered by a continuous layer of smaller cells, as 

 though by an epithelium. This skin-coat, or epithelium, is 



