CHAPTER VII. 



THE CHANGES WHICH TAKE PLACE DURING THE 

 FOURTH DAY. 



ON opening an egg in the middle or towards the end 

 of the fourth day, a number of points in which progress 

 has been made since the third day are at once apparent. 

 In the first place, the general growth of the embryo has 

 been very rapid, so that its size is very much greater 

 than on the previous day. In the second place, the 

 white of the egg has still further diminished, the em- 

 bryo lying almost in immediate contact with the shell 

 membrane. 



The germinal membrane embraces more than half 

 the yolk, and the vascular area is about as large as a 

 halfpenny. 



Corresponding to the increased size of the embryo, 

 there is a great increase in the quantity of blood circu- 

 lating in the vascular area as a whole, though the sinus 

 terminalis is already less distinct than it was. 



The amnion becomes increasingly conspicuous. It 

 is now seen as a distinct covering obscuring to a certain 

 extent the view of the body of the chick beneath, and 



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