CHAPTER VII. 



THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY. 



1. On opening an egg about the middle of the fifth da} 7 , 

 the observer's attention is not arrested by any new features ; 

 but he notices that the progress of development, which was so 

 rapid during the later half of the fourth day, is being con- 

 tinued with undiminished vigour. 



The allantois which on the fourth day began to project 

 from the pleuroperitoneal cavity has grown very rapidly, 

 and now stretches away from the somatic stalk far over the 

 right side of the embryo (which it will be remembered is 

 lying on its left side) in the cavity between the two amniotic 

 folds (Fig. 8, K). It is very vascular, and already serves 

 as the chief organ of respiration. 



The blastoderm has spread over the whole of the yolk- 

 sac ; and the yolk is thus completely enclosed in a bag 

 whose walls, however, are excessively delicate and easily 

 torn. The vascular area extends over about two-thirds of 

 the yolk. 



The splanchnic stalk or umbilical duct has now reached 

 its greatest narrowness ; it has become a solid cord, and will 

 undergo no further change till near the time of hatching. 

 The space between it and the somatic stalk is still con- 

 siderable, though the latter is narrower than it was on the 

 fourth day. 



2. The embryo remains excessively curved, so much so 

 indeed that the head and the tail are nearly in contact. 



The limbs have increased, especially in length ; in each 

 a distinction is now apparent between the more cylindrical 



