CHAPTER V. 



THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY. 



1. OF all days in the history of the chick within the egg 

 this perhaps is the most eventful ; the rudiments of so many 

 important organs now first make their appearance. 



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. 



2. The blastoderm has now spread over about half the 

 yolk, the extreme margin of the opaque area reaching about 

 half-way towards the pole of the yolk opposite to the embryo. 



The vascular area, though still increasing, is much smaller 

 than the total opaque area, being in average-sized eggs about 

 as large as a florin. Still smaller than the vascular area is the 

 pellucid area in the centre of which lies the rapidly growing 

 embryo. 



3. During the third day the vascular area is not only a 

 means for providing the embryo with nourishment from the 

 yolk, but also, inasmuch as by the diminution of the white it 

 is brought close under the shell and therefore fully exposed 

 to the influence of the atmosphere, serves as the chief organ 

 of respiration. 



This in fact is the period at which the vascular area may 

 be said to be in the stage of its most complete development ; 



