December 29, 2003

Making Kang A Roo

Eggs and Egg Membranes

In the adaptation to life on land the cleidoic or enclosed egg was a major development. It freed the vertebrates, from the reptiles onward, from the need to return to water to lay eggs. It also provided sufficient recources for the young to become more or less independent at hatching. The basic feature of the developing cleidoic egg is a series of compartments: amnion, yolk sac and allantois. The amnion grows over the embryo and encloses it in a fluid-filled cushioning sac. The yolk sac is an outgrowth of the embryonic gut and encloses the yolk, while another outgrowth the allantois, store metabolic waste products. Both the yolk sac and the allantois can become partly vascularised and involved in the exchange of respiratory gases. In viviparous mammals the yolk sac and/or the allantois become placentae and involved in exchange of nutrients between the foetus and the mother.

The eggs of kangaroos and other marsupials have a full set of shell membranes, as found in reptiles and birds, but they are much reduced. The yolk is also greatly reduced resulting in very small eggs, only about 0.12 mm in diameter. The eggs of placentals tend to be smaller still, although they overlap the marsupial range. In kangaroos the shell membranes, including the outer tertiary membrane of keratinous protein, persist for much of pregnancy, rupturing only in the last third of the gestation period. Placentals have eliminated the outer shell membrane. The significance of the persistent outer shell membranes in marsupials is undecided, but the suggestion that it provide a necessary immunoprotective barrier separating foetal tissues from maternal tissue is now accepted.

Posted by Madfish Willie at December 29, 2003 01:45 AM | TrackBack
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