oh. vin EVOLUTION OF AMNION 477 



(2) The amniotic fold consisted at first of yolk-sac wall, the body 

 of the embryo being forced down into the yolk-sac as it increased in 

 size, possibly by the resistance of the rigid protective shell associated 

 with the assumption of a terrestrial habit. 



The tendency towards predominant development of the anterior 

 portion of the amniotic fold may probably be correlated with the 

 predominant growth and ventral flexure of the head end of the body 

 which would cause it to dip down into the yolk-sac particularly 

 markedly. 



The delay in the appearance of mesoderm in the region of the 

 proamnion may similarly have been originally due to the pressure of 

 the downwardly flexed head. 



(3) The yolk-sac with its richly developed superficial network of 

 blood-vessels was the respiratory organ of the embryo at this early 

 phase in the evolution of the Amniota. It follows that the portion of 

 it nearest the shell, and therefore in the most favourable position for 

 carrying on the breathing function, would tend to increase in area 

 and would therefore bulge more and more over the body of the 

 embryo (amniotic fold, Fig. 216, A, B, a.f) so as eventually to utilize 

 the whole of the inner surface of the shell. 



(4) The egg being now terrestrial the excretory poisons produced 

 by the activity of the already functional renal organs could no longer 

 pass away by diffusion into the surrounding water. It would obvi- 

 ously be disastrous were they to accumulate in the space round the 

 embryo and they therefore had to be retained within the body. This 

 led to the great and precocious enlargement of the receptacle for 

 these excretory poisons, already present in the pre-amniote ancestor, 

 the allantoic bladder. 



(5) This precocious enlargement of the allantois in turn necessi- 

 tated the early increase in size of the coelomic cavity to accom- 

 modate it. 



(6) The allantoic wall — a part of the gut-wall — was naturally 

 vascular, like the rest of the gut-wall, and with its great increase in 

 size it would come in contact with the inner surface of the somato- 

 pleure. But as soon as it did this respiratory exchange would take 

 place between its blood — through the substance of the somatopleure 

 — and the medium outside. The allantois would thus constitute a 

 new, though at first small, breathing organ. 



(7) As the embryo grew its respiratory needs would grow also. 

 Meanwhile of its two respiratory organs the one — the yolk-sac — 

 would be shrinking in size and therefore diminishing in efficiency. 

 while the other — the allantois — would be increasing in size as it became 

 more and more distended. This would lead to the supplanting of the 

 yolk-sac by the allantois as the main respiratory organ. As the 

 allantois increased in size it would tend to extend in the position of 

 greatest respiratory efficiency, i.e. close under the somatopleure. 



(8) With the development of the allantois and coelome the 

 splanchnopleure would be freed from the somatopleure and the 



