. The development of the chick; an introduction to embryology . {)resented in Fig. 128 are more advanced. The albii-men-sac is closing; its connection with the cavity ofthe amnion by way of the sero-anmiotic connectionwill l)e obvions. The inner wall of the allantois hasfused extensively with the amnion. The lunbilicus ofthe yolk-sac is much reduccnl. and some yolk protrudesinto the albumen (sac of the yolk-sac vnnbilicus). In the outer wall there are three layers, viz., an internal epi-thelial layer, formed by the entoderm of the allantois; a thickvery vascular middle or mesodei-mal layer, fo
. The development of the chick; an introduction to embryology . {)resented in Fig. 128 are more advanced. The albii-men-sac is closing; its connection with the cavity ofthe amnion by way of the sero-anmiotic connectionwill l)e obvions. The inner wall of the allantois hasfused extensively with the amnion. The lunbilicus ofthe yolk-sac is much reduccnl. and some yolk protrudesinto the albumen (sac of the yolk-sac vnnbilicus). In the outer wall there are three layers, viz., an internal epi-thelial layer, formed by the entoderm of the allantois; a thickvery vascular middle or mesodei-mal layer, foi-med by fusion ofthe mesoblast of allantois and choi-ion; and a thin, outer, ecto-dermal layer derived from the chorion. EMBRYO AND EMBRYONIC MEMBRANES 221 Rate of Growth of the Allantois. As the embryo lies on itsleft side, the allantois grows out on the right side of the embryo(Figs. 127 and 130 A) and unites with the chorion about theone hundredth hour. It then spreads rapidly as a flattened sacover the embryo, increasing the extent of the fusion with the. Fig. VM). — Diagrams showing the relations of the allantois,represented by the tinted area, at different ages. (AfterHirota.) Alb., Albumen. Alb. S., Edge of albumen-sac. All. V.,Allantoic vein. am. C, Amniotic cavity. , Sero-amni-otic connection. Y. S., Yolk-sac. A. At 120 hours showing only the amniotic cavity and al-lantois X 2. B. At 144 hours, showing only the anmiotic cavity and al-lantois X C. At 192 hours; the entire yolk x .66. The dotted out-line represents the amniotic cavity. D. At 214 hours. The entire egg after removal of the shell,X .66. The albumen mass is at the left; the albumen-sac is be-ginning to form. chorion, hence of its outer wall pari pasf<u. At the end of tliefifth day it covers more than half of the embryo (Fig. 130 A);at the end of the day the embryo is entirely covered by 222 THi<] l)]:\i<:lopi\iext of the chick the allantois (Fi,<;-. MA) B); at the end of the
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