ALLANTOIS 



581 



slightly later stage, transverse section ; C, stage with completed amnion and 

 commencing allantois ; D, stage in which the allantois has hegun to envelop the 

 embryo and yolk-sac. The ectoderm is represented by a blue, the endoderm by 

 a red line ; the mesoderm is grey. 

 all. allantois ; all' . the same growing round the embryo and yolk-sac ; am. amnion ; 

 am./, amniotic fold ; an. anus ; Or. brain ; cirl. co^lome ; ca:l' . extra-embryonic 

 ccelome ; hi. heart; ins.ent. mesenteron; iiith. mouth; nch. notochord ; sp.cd. 

 spinal cord; sr.m. serous membrane ; mnh.d. umbilical duct ; 7';'. 7». vitelline 

 membrane ; yk. yolk-sac. (Reduced from Parker and Haswell's Zoology.') 



having the precise anatomical relations of the urinary bladder 

 of the Frog. It increases rapidly in size (Figs. 155 and 154, 

 a//), and makes its way, backwards and to the right, into the 

 extra-embryonic coelomej between the amnion and the serous 



cnth 



7/ClSC 



cUl 



Fig. 154. — Egg of fowl, at the sixth day of incubation, with embryo and fa;La 

 appendages. 

 a. air-space ; all. allantois ; am. amnion ; ar. vase, area vasculosa ; ciub. embryo ; 

 yk. yolk-sac. (From Parker and Haswell's Zoology^ after Duval.) 



membrane (Fig. 153, C, D). Arteries pass to it from the 

 dorsal aorta, and its veins, joining with those from the yolk-sac, 

 take the blood through the liver to the heart. Next, the 

 distal end of the sac spreads itself out and extends all round 

 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- 



