CHAPTER V 



THE DEVELOPMENT OF THE THIRD DAY 



SINCE there is no especial advantage in 

 dividing the third day into two periods, 

 as was done with the second day, it will 

 be treated as a single period. 



Of all the twenty-one days of the chick's 

 embryonic development, the third is, perhaps, 

 the most eventful in the number of structures 

 that make their appearance. 



There is a marked increase in the size of 

 the embryo and of the blastoderm during this 

 day, the blastoderm now covering about half 

 of the surface of the yolk. There may also be 

 noticed a corresponding decrease in the amount 

 of the white of the egg, due either to its direct 

 absorption by the blood vessels of the vascular 

 area, or to the absorption of the yolk which is, 

 in turn, replaced by the white. 



The diminution of the white brings the vas- 

 cular area close to the inner surface of the 

 shell membrane, so that it is now possible for 



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