GENERAL ACCOUNT. 



227 



of the brain vesicles and of the eyes. The limbs are still small, 

 but have increased considerably in size as compared with the 

 earlier stages, and already show indications of their division into 

 segments (Fig. 115). From the under surface of the tail of the 

 embryo a saccular diverticulum, with thin but very vascular 

 walls, arises as an outgrowth from the alimentary canal : this 

 is the allantois (Fig. 115, TA), a structure which grows very 



.AN 



FIG. 101. The Hen's Egg at the end of the ninth day of incubation, seen in 

 vertical section. The embryo naturally lies with its left side on the yolk- 

 sac, but has been lifted up in order to show its shape more clearly, x f. 



AN", inner or 'true' auniion. HM, liyoinaiiilibular cleft. SV air chamber. TA 

 ullantois. "WA, white or albumen. YS, yolk-.sac. 



rapidly during the succeeding days, and forms the respiratory 

 organ of the embryo. 



By the end of the ninth day the embryo has grown consi- 

 derably, and has attained the shape and proportions shown in Fig. 

 101. The body walls are now definitely formed, and rudiments 

 of the feathers are already present. The head is still dispropor- 

 tionately large, and the eyes are of enormous size. The beak, 

 which was absent in the earlier stages, has now grown out from the 

 front of the face, and at once gives the head a characteristic 

 avian appearance. The neck is long and slender. The body is 

 much more bulky than before, largely owing to the great size of 



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