126 EVOLUTION OF THE EARTH 



anatomically from an amphibian, but with certain forms, such 

 as Cacops (Fig. 22) from the Permo-Carboniferous red beds 

 of Texas, the distinction is by no means so clear, for such as 

 this were transitional, Cacops being yet on the amphibian side 

 of the frontier. 



The change of habit induced by this aridity meant a radical 

 change in the character of the vertebrate egg, with the develop- 

 ment of certain membranes (Fig. 23) whereby the contained 

 young could respire air, unnecessary in the water-laid egg. 

 Thus the terrestrial egg, typified by that of bird or reptile, is 

 large, with an abundance of nutrient "yolk" for the developing 

 embryo, and enclosed in a protective though porous shell. 

 Two membranes are developed, the first of which, the amnion, 

 is merely protective, in that the cavity between its two layers 

 contains a fluid which serves to absorb mechanical shocks and 

 to guard the embryo against too rapid temperature changes. 

 The other membrane is known as the allantois (see Fig. 23). 

 This, when fully developed, lines the inside of the shell and 

 is abundantly supplied with blood-vessels, veins, arteries, and 

 capillaries, which connect with the embryonic circulation. 

 This is the respiratory device, for the blood coursing through 

 the allantoic capillaries is thus in close osmotic relationship 

 with the air which penetrates the porous shell, and thus the 

 blood is purged of its carbonic acid gas and other volatile 

 impurities and receives its life-sustaining supply of oxygen. 

 Such an egg must be laid on land or retained within the mother 

 until it hatches, for the contained young would drown if the 

 egg were submerged, as surely as would the adult. 



But the development of these membranes has a still deeper 

 significance, because the allantois through an added function 

 becomes the intermediary between the unborn young and its 

 mammalian mother, and hence its development, due to that 

 far-off climatic change, has rendered possible the higher 

 (placental) mammal with all which that implies to humanity. 



