CHAPTER VIII. 



THE CHANGES WHICH TAKE PLACE ON THE FIFTH 



DAY. 



ON opening an egg about the middle of the fifth 

 day, the observer's attention is not arrested by any new 

 features ; but he notices that the progress of develop- 

 ment, which was so rapid during the later half of 

 the fourth day, is being continued 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. 9, 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 vitelline duct has now 

 reached its greatest narrowness ; it has become a solid 



