134 THE EVOLUTION OF MAN. 



development of the human embryo; all the many peculiari- 

 ties in the development of the organs which we shall 

 presently notice in detail ; and, finally, the chief special 

 arrangements of the internal structure of the body in all 

 fully developed Amnion Animals; all these so clearly demon- 

 sti'ate the common origin of all Amnion Animals from a 

 single extinct parent-form, that it is . impossible to conceive 

 their origin as polyphyletic, and that they originated from 

 several independent parent-forms. This unknown common 

 parent-form is the Primitive Amnion Animal (Protam- 

 nion). In external appearance the Protamnion was most 

 probably an intermediate form between the Salamanders 

 and the Lizards. 



It was probably during the Permian Period that the 

 Protamnion originated ; perhaps at the beginning, perhaps 

 at the close of that period. This we know from the fact 

 that the Amphibia did not attain their full development till 

 the Carboniferous Period, and that toward the close of the 

 Permian the first fossil Reptiles make their appearance — 

 or, at least, fossils {_Proterosaurus, Rhopalodon) which must 

 in all probability be referred to lizard-like Reptiles. Among 

 the great and pregnant modifications of the vertebrate 

 organization determined during this period by the develop- 

 ment of the first Amnion Animals from salamander-like 

 Amphibians, the three following are especially important : 

 the total loss of water-breathing gills and modification of 

 the gill-arches into other organs ; the formation of the 

 allantois, or primitive urinary sac; and, finally, the develop- 

 ment of the amnion. 



The total loss of the respiratory gills must be regarded 

 as one of the most prominent characters of aU Amnion 



