582 ALLANTOIS chap. 



mains of the albumen, which has, by this time, been largely 

 absorbed. The allantois serves as the embryonic respiratory 

 organ, gaseous exchange readily taking place through the 



Fig. 155. — Chick at the fifth day of Incubation. 

 all. allantois ; mn. cut edge of amnion ; ait.s. auditory sac ; f. hr. fore-brain ; f /. 

 fore-limb ; h, br. hind-brain ; h. I, hind-limb ; ht. heart ; hy. hyoid arch ; w. br. 

 mid-brain ; w«. mandibular arch ; t. tail. (From Parker and Haswell's Zoology^ 

 after Duval.) 



porous shell; its cavity is an embryonic urinary bladder, 

 excretory products being discharged into it from the 

 kidneys. 



At the end of incubation the embryo breaks the shell by means of a 

 little horny elevation or caruncle at the end of the beak. By this time 

 the remainder of the yolk-sac has been drawn into the coelome, and the 

 ventral body-walls have closed round it. On the shell being broken 

 the allantois gradually shrivels up, respiratory movements begin, the 



