412 ZOOLOGY SECT, xm 



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 off from the yolk the 

 rudiment of one of the two characteristic embryonic membranes, 

 the amnion, has appeared. A crescentic amniotic fold arises 

 (Fig. 1016, A, am./.), in front of the head-end of the embryo, from 

 the region of the pro-anmion : 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. /'.) 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 cceloine. 



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 (ST. 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 mesoderm 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. 1014, A, all.). It increases rapidly 

 in size (Fig. 101 5, all.), arid makes its way, backwards and to the right, 

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

 membrane (Fig. 1016, 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 



