. Animal biology. Zoology; Biology. 376 METAZOAN PHYLA them from drying and also need to be safeguarded from mechanical injuries. Certain coverings of the egg meet the former need, and an amniotic sac the latter. 407. Egg.—Three coverings are added to the egg cells as adaptations to developm^ent in a non-aquatic environment (Fig. 255). These are (1) a layer of alhumen, which provides protection against drying and mechanical injury and also serves as food for the embryo; (2) an egg membrane, which in some cases becomes thick and leathery and to which may be added lime; and (3) a shell, present
. Animal biology. Zoology; Biology. 376 METAZOAN PHYLA them from drying and also need to be safeguarded from mechanical injuries. Certain coverings of the egg meet the former need, and an amniotic sac the latter. 407. Egg.—Three coverings are added to the egg cells as adaptations to developm^ent in a non-aquatic environment (Fig. 255). These are (1) a layer of alhumen, which provides protection against drying and mechanical injury and also serves as food for the embryo; (2) an egg membrane, which in some cases becomes thick and leathery and to which may be added lime; and (3) a shell, present in many reptiles and normally in all birds, composed entirely of lime. The albumen, the membrane, and the shell are all secreted by glands lying along the course of the oviduct in the order in which the envelopes which they form are added. vitelline membrane Germinal area Chalaza. Chalaza Albumen ^Egg shell Egg membrane White YeJIo w yolk yolk Fig. 255.—Diagrammatic section of a hen's egg. 408. Amnion.—Since the eggs of reptiles and birds are meroblastic, discoidal cleavage occurs and a sheet of cells, the blastoderm, is formed. From a part of the blastoderm is developed the embryo, and the rest of it grows around and completely envelops the yolk. The blastoderm splits, forming two layers, ectoderm outside and entoderm inside, next to the yolk. Between these appears a third layer, the mesoderm, and this also splits into two layers, one of which, the somatic layer, lies next to the ectoderm and with it forms the somatopleure, the other, the splanchnic layer, next to the entoderm and with it forms the splanch- nopleure. Between these two mesodermal layers is the coelom (Fig. 256 B). The amnion is a fold of the blastoderm outside the area forming the embryo and is composed of two layers (Fig. 256 C), ectoderm and somatic mesoderm, or somatopleure. As this fold grows up around the embryo it meets above and incloses a sac, known as the amniotic sac, which surrounds the embryo
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