440 



ZOOLOGY 



SECT. XIII 



while the metatarsal region elongates and gives rise to the equally 

 characteristic foot. At the same time feather-papillae make their 

 appearance, arranged in narrow and well-defined pterylse. 



At an early period capillaries appear in the extra-embryonic 

 blastoderm between the opaque and pellucid areas, and give rise 



e-mb 



Z'OSC 



cUl 



Fig. 1077. —Gallus bankiva. Egg with embryo and fcetal appendages, a. air-space; oXL 

 allantois ; am, apinion ; ar. vam. area vasculosa ; ejjit. embryo ; ylc, yolk-sac. (After Duval.) 



to a well-defined arm vasculosa (Fig. 1077, ar. vase.) : they are sup- 

 plied by vitelline arteries from the dorsal aorta, and their blood is 

 returned by vitelline veins which join the portal vein and take the 

 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 mcnibranes — 

 the amnion, has appeared. A crescentic amniotie fold arises 

 (Fig. ] 078, A, am. /.), 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./.') it consists 

 from the first of ectoderm ^j/tjs 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 coelome. 



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 protecti\e water-cushion to the enclosed embryo. 



