CH. XI 



ALLANTOIS 



S8i 



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

 commencing allantois ; D, stage in which the allantois has begun 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 ; alf. the same growing round the embryo and yolk-sac ; am. amnion ; 

 am./", amniotic fold ; an. anus ; br, brain ; ccel. ccelome ; caP. extra-embryonic 

 coelome ; kt. heart; tns.eni. mesenteron ; mtk, mouth; nch. nolochorA ',sp.cd. 

 spinal cord ; sr, ni. serous membrane ; U7nb. d. umbilical duct ; vt. vi. 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, 

 all)^ and makes its way, backwards and to the right, into the 

 extra-embryonic coelome, between the amnion and the serous 



ixm 



e^mb 



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«-- 



all 



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

 appendages. 

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



