CHAP, xin ALLANTOIS 609 



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./, amniotic fold ; an. anus ; br. brain ; ccel. coelome ; cod', extra-embryonic 

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

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

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



derm externally, is the amnion (am, and Fig. 164, am'), the 

 cavity enclosed by which becomes filled with a watery 

 amniotic fluid, serving as a protective water-cushion to the 

 contained embryo. Its outer layer, formed of ectoderm 

 externally and mesoderm internally, is the serous mem- 

 brane, sometimes spoken of as ihe false amnion (sr. m, am"} : 

 this comes to lie just beneath the vitelline membrane 

 (p. 565), with which it subsequently fuses. 



The second of the embryonic membranes, the allantois, 

 is developed as an outpushing of the ventral wall of the 

 mesenteron near its posterior end (Figs. 165, C, all, and 

 166), and consists, therefore, of a layer of visceral meso- 

 derm lined by endoderm (splanchnopleure, p. 580). It 

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

 precise anatomical relations of the urinary bladder of the 

 frog. Increasing rapidly in size, it makes its way, back- 

 wards and to the right, into the extra-embryonic coelome, 

 between the amnion and the serous membrane (C, D). 

 Allantoic arteries pass to it from the dorsal aorta, and its 

 veins, joining with the vitelline veins from the yolk-sac, 

 take the blood through the liver to the heart (p. 600). 

 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 thus coming to lie immediately beneath the shell- 

 membrane. It finally encloses the whole embryo and 

 yolk-sac, together with the remains of the albumen, 

 which has, by this time, been largely absorbed (Fig. 

 159, D). The allantois serves as the embryonic respira- 



