CHAPTER XII 



AMNIOTA 



Reptiles and Birds 



The remaining groups of vertebrates have become purely terrestrial : 

 they have completely emancipated themselves from the primitive aquatic 

 environment. In rendering this possible a great part has been played 

 by certain reproductive arrangements which constitute a common 

 character linking them together into a group known as the Amniota. 

 The essential feature is that the time of hatching is delayed until those 

 stages which in an amphibian would be passed through in the water 

 have been completed in the seclusion of the egg. This has been rendered 

 possible by a number of adaptive modifications : 



(i) The egg is enclosed in a hard or tough protective shell, 



(2) The egg contains an abundant supply of stored-up food-material 

 or yolk which will serve for the nourishment of the embryo during the 

 prolonged period before hatching, 



(3) The body of the developing embryo comes to be enclosed in a 

 water-jacket known as the amnion which serves to protect its soft and 

 delicate body from the dangers of knocks and jars, and 



(4) In order to accommodate the poisonous urinary secretion, which 

 is no longer able to diffuse away into the surrounding medium, there 

 takes place an immense hypertrophy of the allantois, and, incidentally, 

 the vascular wall of this organ lying close under the egg-shell is able to 

 meet the respiratory needs of the embryo. 



The detailed description of these developmental features will be 

 reserved for Chapter XIV. 



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