OBSERVATIONS ON THE LIVING EGG. 485 



The cavity of the middle ear and Eustachian tube develops 

 in part from the first or hyomandibular visceral cleft. 



The succeeding changes in the egg are as follows : 



Fourth day. The egg albumen becomes restricted to a 

 small area, which is gradually enclosed by the allantois. 



The wings and legs make their appearance as flattened 

 conical buds. The internal changes described during the 

 third day are extended. The permanent kidney and its duct, 

 the ureter, are developed. 



Fifth day. The embryo is so strongly flexed that the 

 head and tail are almost in contact. The cartilaginous 

 elements of the skeleton have appeared. 



Sixth day. The characters of a bird are developed in 

 the wings and feet, skull, and alimentary canal. 



Seventh day. The amnion and allantois commence 

 rhythmical contractile movements by which the embryo is 

 rocked to and fro. The head now ceases to grow more rapidly 

 than the body. 



Ninth day. Feathers begin to protrude through the skin, 

 though still enclosed within their sheaths. 



Twentieth day. The beak perforates the inner shell- 

 membrane and the chick breathes air. The allantois circula- 

 tion ceases and that of the lungs begins. 



Twenty-first day. The chick is hatched. 



III. OBSERVATIONS ON THE LIVING EGG 

 Place an egg of the desired age in a dish of warm salt-solu- 

 tion, allowing it to rest on the plaster mould and just covering 

 with fluid. Carefully remove the shell over the uppermost area 

 so as to make a window an inch or an inch and a half in diameter. 

 Allow some of the egg-white to escape into the dish, but take great 

 care to avoid puncturing the yolk. The blastoderm will be found 

 lying just under the aperture, and if the egg has developed nor- 

 mally the following structures will be visible and should be 

 drawn, using a hand-lens when necessary (figs. 85-94). The 

 large end of the egg should be drawn pointing to the left of the 

 observer. 



End of the first day (fig. 85). The neural folds- are well 



