REPTILIA 



213 



The amnion (Fig. 122), 

 which is formed very early, 

 results from the fusion to- 

 gether of two lateral folds of 

 the extra-embryonic blas- 

 toderm and is a sort of 

 bladder-like inembrane con- 

 taining a watery fluid in 

 which the growing embrj^o 

 hes. The fluid within the 

 amnion increases greatly in 

 amount and provides an 

 ample space for the further 

 growth of the embryo, and 

 preserves the latter from con- 

 tacts, mechanical abrasions, 

 or other mechanical injuries. 



The aUantois (Fig. 122) 

 starts as a finger-like diver- 

 ticulum of the embryonic w 

 hind-gut and grows out ex- ' 

 tensively l)etween the amnion 

 and the chorion as a large 

 umbrella-shaped sac, which 



is highly vascular; and it p^^ 122. -Diagram., of the embryonic 

 functions as an embryonic membranes, amnion, allantoi.s, yolk-sac, of 

 lung. The amnion and the Ammotes. A, Sauropsida (reptiles and birds). 



allantois too-ether make de- ^' ™^™"^^' ''''^^ primitive allantoic pla. 

 ananrois togetner make ae ^^^^^ ^^p^^^ LuU, after Wilder.) 



velopment possible even in 



arid regions and give to the reptiles a decided advantage in avail- 

 able range, since they may live far from the water. 



The Earliest Reptiles and Their Origin 



"Just when the animals we call reptiles arose in geological history 

 we do not know; certainly it was in early Penn.s\'lvanian (Upper Car- 

 boniferous) times, probably Mississippian (Lower Carboniferous). 

 That they arose from what we call the Amphibia, forms with tcmno- 

 spondylous vertebrffi, is certain, though there is not much more reason 

 for calling the ancestral stock Amphibia than Reptilia. I prefer to 



