THE EVOLUTION OF THE REPTILES 437 



surrounded by upgrowths of the blastoderm forming the 

 amnion and enclosing the amniotic cavity. The embryo 

 developed in the fluid contents of this cavity, which may thus 

 be regarded as an artificial enclosed pond. The food require- 

 ments of the developing embryo were met as in lower forms by 

 a store of yolk in the yolk-sac. This entailed no new modifi- 

 cations by itself, but as embryonic development took a longer 

 time, the quantity of yolk in the egg was relatively greater and 

 this necessitated the modification of the process of gastrula- 

 tion, and the formation of a primitive streak. There remained 

 the difficulty of breathing, for although the gill-slits were 

 developed they opened into the amniotic cavity, the oxygen- 

 content of which could not be renewed. The problem was 

 solved by the development of the allantois, representing the 

 bladder of the amphibia. The allantois became applied to 

 the inner surface of the porous shell, and as it was highly 

 vascularised, the respiratory exchange took place in it. At the 

 same time, the allantois served as a receptacle for the non- 

 volatile excretory products of the embryo during development. 

 Because of these structural adaptations, the reptile does not 

 pass through a metamorphosis, but hatches from the egg as a 

 more or less perfect miniature replica of the. adult. 



In the reptiles, the head is capable of extensive independent 

 movement, and a definite neck is formed. In this connexion, 

 the two first vertebrae become modified into the atlas and the 

 axis. 



The most primitive known reptile is Seymouria (Permian), 

 and it is remarkable for the fact that its characteristics are not 

 intermediate between those of amphibia and of reptiles, but 

 some of its characters are frankly .amphibian and others rep- 

 tilian. Seymouria is therefore a mosaic transitional form. It 

 is probable that the transition from amphibia to reptiles took 

 place in the Carboniferous. 



Seymouria belongs to the group of Reptiles known as 

 the Cotylosaurs, and they preserve the complete covering of 

 dermal bones over the skull which they inherited from their 

 Stegocephalian amphibian ancestors. The nature of the skull 

 is of importance in tracing out the lines of evolution of the 



