€48 ZOOLOGY. ‘ 
Fig, 541.—Five schematic figures showing the development of the foetal egg-mem- 
branes, where in all except the last the embryo is represented as if seen in longitudinal 
section. 1. Diagram of egg with zona pellucida, plastoderm (@, 4), poole disk, and) 
embryo, 2. g with the first traces of the yolk-sac (@) and amnion (ks, ss, and am. 
3. Egg with the amnion uniting and forming a sac; the allantois (ad) budding out. 
4. Ege with the villi of the serous membrane (sz); the allantois larger; embryo with 
mouth and anal opening. 5. Egg in which the vascular layer of the allantois lies close 
to the serous layer and has grown into the villi of the same, constituting the true 
chorion (ch). Yolk-sac much smaller, about to be drawn into the cavity of the amnion, 
