428 ZOOLOGY SECT. 



blood, through the liver, to the heart. The vascular area gradually 

 extends, until it covers the whole of the yolk-sac : its vessels take 

 an important share in the absorption of the yolk by the embryo. 



Before the embryo has begun to be folded ofi from the yolk the 

 rudiment of one of the two characteristic embryonic membranes, 

 the amnion, has appeared. A crescentic amniotic fold arises 

 (Fig. 1089, A, am.f.), in front of the head-end of the embryo, from 

 the region of the pro-amnion : it consists at first of ectoderm only, 

 the mesoderm not having yet spread into the pro-amnion. The 

 fold is soon continued backwards along the sides of the body (B) 

 and round the tail (A), but in these regions (am.f.) it consists from 

 the first of ectoderm plus the somatic layer of mesoderm, i.e., it 

 is a fold of what may be called the extra-embryonic body-wall. 

 The cavity is a prolongation of the space between the somatic and 

 splanchnic layers of mesoderm, i.e., is an extension of the extra- 

 embryonic ccelome. 



The entire amniotic fold gradually closes in above (C), forming 

 a double-layered dome over the embryo. Its inner layer, formed 

 of ectoderm internally and mesoderm externally, is the amnion 

 (am.), the cavity of which becomes filled with a watery amniotic 

 fluid, serving as a protective water-cushion to the enclosed embryo. 

 Its outer layer, formed of ectoderm externally and mesoderm in- 

 ternally, is the serous membrane (sr. m.) : it comes to lie just beneath 

 the vitelline membrane, 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 at its posterior 

 end (C, all.), and consists, therefore, of a layer of splanchnic meso- 

 derm lined by endoderm. It has at first the form of a small ovoid 

 sac having the precise anatomical relations of the urinary bladder 

 of Amphibia (Fig. 1087, A, all.). It increases rapidly in size (Fig. 

 1088, all.), and makes its way, backwards and to the right, into 

 the extra-embryonic ccelome, between the amnion and the serous 

 membrane (Fig. 1089, 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 encloses the whole embryo and yolk-sac 

 together with the remains 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 porous shell ; its cavity is an embryonic urinary 

 bladder, excretory products being discharged into it from the 

 kidneys. 



At the end of incubation the embryo breaks the shell, usually by 

 means of a little horny elevation or caruncle at the end of the beak. 



