582 



ALLANTOIS 



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 



«».* 



hhr 



m.hr 



J'-l'^ 



Fig. 155. — Chick :it the fifth day of incubation. 

 all. allantoi^ ; am. cut edge of amnion ; au . s. auditory sac ; y? />>'. fore-brain ; / I. 

 fore-litulj ; h. br. hind-brain ; /i. I. hind-limb ; ht. heart ; hy. hyoid arch ; Jii. br. 

 mid-brain ; ;//;;. 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 embiyo breaks the shell by means of a 

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

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

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

 the allantois gradually .shrivels up, respiratory movements begin, the 



