A HISTORY Of VERTEBRATES: AMPHIBIANS AND REPTILES 



455 



each one by certain oviducal cells. Albumin and similar materials 

 around the egg provide additional food, ions and water. The leathery or 

 calcareous shell serves tor protection against mechanical injury and 

 desiccation, yet it is porous enough to permit gas exchange. Such an egg, 

 which contains or has the means of providing all substances necessary 

 for the complete development of the embryo to a miniature adult, is 

 called a cleidoic egg. Reptiles lay fewer eggs than lower vertebrates, but 

 the eggs are larger, better equipped and laid in sheltered situations, 

 so the mortality is low. A collared lizard lays only four to twenty-four 

 eggs in contrast to the two or three thousand of the leopard frog. 



As the embryo develops, it separates from the yolk, which becomes 

 suspended in a yolk sac (Fig. 23.7). Protective layers of tissue fold over 

 the embryo. The outermost of these is the chorion. An amnion lies 

 beneath it and forms around the embryo a fluid-filled chamber, which 

 serves as a protective water cushion and provides an aquatic environ- 

 ment in which the embryo develops. These two membranes appear to be 

 derived phylogenetically from something similar to the superficial layers 

 covering the yolk sac of certain large yolked fish embryos. Another mem- 

 brane, the allantois, is a saclike outgrowth from the embryo's hindgut. 

 It is homologous to the urinary bladder of the frog, but extends beyond 

 the body wall, passing between the amnion and the chorion. Its highly 

 vascular wall unites with the chorion, and gas exchange with the ex- 

 ternal environment occurs there. Nitrogenous excretory products, largely 

 in the form of crystals of uric acid, accumulate in the cavity of the 

 allantois. 



Yolk sac, chorion, amnion and allantois are collectively called the 

 extraembryonic membranes. These adaptations for terrestrial reproduc- 

 tion are found in the embryos of all reptiles, birds and mammals. These 

 groups of vertebrates are often called amniotes, after one of these mem- 

 branes. In contrast, the various fish groups and amphibians are called 

 the anamniotes. 



''EmhryoTiic iut Chorioa.mn.otic Chorion" 



Coelom .^- --~~/ ~^ ^ ., . ;r^ — ^ ^ /" fold 



^— '- ^ ^—laminar ^ /^^ .X r-v 



Amnion- 



Allanlois 



- ilaminco- 

 A'- ' " °H^''%* • ';'°iv*^ — yolk sac 

 ■'^^t^iis:'^ — Ectoderm -^— -..'■ 

 ^ Mesoderm 

 A — Endoderm. B *-• 



Figure 23.7. Sections of vertebrate embryos to show the extraembryonic mem- 

 branes. A, The trilaminar yolk sac of a large yolk fish embryo consists of all three germ 

 layers. B, The chorioamniotic folds of an early embryo of a reptile appear to have 

 evolved from the ectoderm and part of the mesoderm of a trilaminar yolk sac. C, A 

 later reptile embryo in which the extraembryonic membranes are complete. Notice that 

 the yolk sac is bilaminar. The albumin and shell, which surround the reptile embryo 

 and extraembryonic membranes, have not been shown. 



