ALLANTOIS 



599 



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

 am.f., am.fl) amniotic fold; an. anus; br. brain; cceL ccelome ; ccel'. extra-embryonic 

 coelome ; ht. heart ; ms. ent. mesenteron ; wth. mouth ; nch. notochord ; sp. cd. 

 spinal cord ; sr. m. serous membrane ; umb. d. umbilical duct ; vt. m. vitelline 

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



is developed as an outpushing of the ventral wall of the 

 mesenteron at its posterior end (c, a//), and consists, 

 therefore, of a layer of visceral mesoderm lined by endo- 

 derm. It has at first the form of a small, ovoid sac having 



FIG. 165. Egg of fowl, at the sixth day of incubation, with embryo and foetal 



appendages. 



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

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



the precise anatomical relations of the urinary bladder of the 

 frog. It increases rapidly in size (Figs. 166 and 165, all\ 

 and makes its way, backwards and to the right, into the 

 extra-embryonic ccelome, between the amnion and the serous 

 membrane (Fig. 164, 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 



