CHAPTER VI. 



THE CHANGES WHICH TAKE PLACE DURING THE FOURTH 



DAY. 



1. 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 embryo 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 circulating in the 

 vascular area as a whole, though the sinus terminalis is 

 already less distinct than it was. 



2. 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 all traces of 

 the junction of its folds are by this time lost. As yet there 

 is very little fluid in the amniotic sac proper, so that the 

 true amnion lies close upon the embryo. 



3. The folding off of the embryo from the yolk sac has 

 made great progress. The splanchnic stalk, which on the 

 third day was still tolerably wide, inasmuch as about one 

 third of the total length of the alimentary canal was as yet 

 quite open to the yolk sac below, now becomes so much con- 

 stricted by the progressive closing in of the splanchnopleure 



