W F B 3000 Lecture Notes - Lecture 6: Amniotic Sac, Amniote, Yolk Sac
19 Oct 2017
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Copulatory organs are not usually visible - stored inside cloaca. Amnion (amniotic sac): membrane surrounding embryo; filled with amniotic fluid. Yolk sac: food for embryo; shrinks as embryo grows. Allantois: structure that assists with gas diffusion and waste storage; expands as embryo grows. Chorion : thin outer membrane; immediately inside shell. Amniotes: the group of tetrapods that includes reptiles, birds, and mammals. But mammals (and some reptiles) don"t actually lay eggs the earliest mammals didand a few still do. In placental mammals: the structures of a typical amniotic egg is slightly modified -more later. Most reptiles have oviparous birth: young hatch from eggs with a shell outside mom"s body. Shells are porous to allow for gas exchange. A few snakes/lizards have viviparous birth: live birth. A few snakes have ovoviviparous birth: eggs develop but are retained in mom"s body- hatch inside- looks live birth. Many reptiles (all snakes) have genetic sex determination- like us.
