CHAPTER VI. 



THE CHANGES WHICH TAKE PLACE DURING THE THIRD 



DAY. 



Oe all days in the history of the chick within 

 the egg this perhaps is the most eventful; the rudi- 

 ments of so many important organs now first make their 

 appearance. 



In many instances we shall trace the history of these 

 organs beyond the third day of incubation, in order to 

 give the reader a complete view of their development. 



On opening an egg on the third day the first thing 

 which attracts notice is the diminution of the white of 

 the egg. This seems to be one of the consequences of 

 the functional activity of the newly-established vascular 

 area whose blood-vessels are engaged either in directly 

 absorbing the white or, as is more probable, in absorbing 

 the yolk, which is in turn replenished at the expense of 

 the white. The absorption, once begun, goes on so 

 actively that, by the end of the day, the decrease of the 

 white is very striking. 



The blastoderm has now spread over about half 

 the yolk, the extreme margin of the opaque area reach- 



