6oo 



ALLANTOIS 



the embryo and yolk-sac (D, all'\ fusing, as it does so, 

 with the serous and vitelline membranes, and so coming to 

 lie immediately beneath the shell-membrane. It finally en- 

 closes the whole embryo and yolk-sac, together with the re- 

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

 absorbed. The allantois serves as the embryonic respiratory 



Tit 



m.ltr 



7t.l 



FIG. 166. Chick at the fifth day of incubation. 



all. allantois ; am. cut edge of amnion ; au. s. auditory sac ; f. br. fore-brain ; f L 

 fore-limb ; h. br. hind-brain ; h. 1. hind-limb ; ht. heart ; hy. hyoid arch ; m. br* 

 mid-brain ; mn. mandibular arch ; t. tail. (From Parker and Harwell's Zoology, 

 after Duval.) 



organ, gaseous exchange readily taking place through the 

 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 



