206 GENEEATION AND DEVELOPMENT. 



day, the wings and legs are just recognizable as outgrowths, 

 and the allantois, an embryonic sac destined to serve as a 

 respiratory organ beneath the shell, begins to grow rapidly ; 

 on the sixth day, the allantoic blood-vessels are clearly seen 

 as well as the vitelline blood-vessels, while the embryo has 

 greatly increased in size. The appearance is much the 

 same on the seventh day, but the embryo is still larger, 

 and the same may be said of the appearance on the eighth 

 day, a noticeable feature being, however, the prominence 

 and large size of the eyes. 



The above represents, without entering into details, the 

 course of development up to and including the eighth day. 

 It is evident that, although Aristotle's description, previously 

 given, is not quite clear, he refers to the vitelline or yoke 

 sac and the vitelline blood-vessels, and also to the allantois 

 and the allantoic blood-vessels. Fig. 9 represents an egg 

 with the shell carefully removed from part of one side to 

 show the allantoic blood-vessels (in full lines,) and the 

 vitelline blood-vessels (in broken lines), at the eighth day, 

 which seems to correspond with the time of the latest stages 

 in Aristotle's description, so far as it has been given above. 



He next describes the appearance observable on the 

 tenth day, and his description shows that he made a very 

 careful examination of the egg at this period of incuba- 

 tion. 



If the young chick be removed on the tenth day, and 

 freed from the amnion and yolk sac, it will be seen to show 

 an'"abnormally large head and large eyes, a short beak, and 

 fairly well developed legs and wings, as shown in Fig. 10. 

 By placing the young chick in water in a test tube and 

 holding it towards the light, numerous feathers with their 

 barbs and shafts can be seen by means of a lens. Aristotle 

 says that, on the tenth day, the entire bird and its parts are 

 distinct, its head being seen to be larger than the rest of its 

 body, and its eyes larger than the rest of the head ; if re- 

 moved, he says, its eyes will be found to be black. At a 

 later stage, he proceeds to say, the chief viscera {i. e., the 

 heart, liver, &c.) are visible, and also the gizzard and 

 intestines, while the blood-vessels from the heart appear to 

 extend to the yolk stalk. He also describes, in greater 

 detail than before, the embryonic membranes, showing that 

 he had examined the allantois, lying beneath the shell 

 membrane, the yolk sac, and the amnion which, he says, is 



