THE REPTILIA. (57 



their lateral continuations at length meet over the middle line 



of the body, and there coalesce: so that the body is covered for 

 a while by a double sac, the inner layer of which is formed by 

 that wall of the fold of the serous layer which is inferior, or 

 nearest to the body of the embryo ; while the outer layer is 

 formed by that wall which is superior, or furthest from the body 

 of the embryo. The outer layer eventually disapj^ears as a dis- 

 tinct structure, while the inner remains as the amnion. From 

 the mode of formation which has been described, it results that 

 the amnion is a shut sac, enveloping the body of the embryo ; 

 and is continuous, on the ventral side of the body, with the in- 

 tegument of a region which eventuallv becomes the umbilicus 

 (Fig. 32, F). 



The allantois is developed much later than the amnion, 

 neither from the serous nor from the mucous (or epidermic and 

 epithelial) layers of the germ, but from that intermediate stratum 

 whence the bones, muscles, and vessels are evolved. It arises, 

 as a solid mass, from the under part of the body of the embryo, 

 behind the primitive intestinal cavity ; and, enlarging, becomes 

 a vesicle, which rapidly increases in size, envelopes the whole 

 embryo, and, being abundantly supplied with arterial vessels 

 from the aorta, serves as the great instrument of respiration 

 during foetal life ; the porosity of the egg-shell allowing the 

 allantoic blood to exchange its excess of carbonic acid for 

 oxygen by osmosis. 



The amnion and the external part of the allantois are thrown 

 off at birth. 



That which has just been stated respecting the development 

 and characters of the amnion and allantois of the chick is true 

 not only of all Birds, but of all Beptilia. 



All embryonic Reptilia are provided with an amnion and an 

 allantois, like those just described in the foetal fowl. In the 

 embryonic state, also, they possess visceral arches and clefts, but 

 no respiratory tufts are ever developed in the arches, nor are 

 reptiles endowed with an apparatus for breathing the air dis- 

 solved in w r ater at any period of their existence. The skull of 

 all Reptilia is articulated with the vertebral column by a single 

 condyle, into which the ossified basi-occipital enters largely 



f 2 



