1^6 THE EVOLUTION OF MAN. 



The formation of the amnion and the allantois, together 

 with the total loss of the gills and the exclusive adoption 

 of lung-respiration, are the most important characters by 

 which all Amnion Animals are distinguished from the lower 

 Vertebrates which we have been considerinor. In addition 

 to these there are a few subordinate characters which are 

 constantly inherited by Amnion Animals, and are altogether 

 wanting in animals without an amnion. One striking em- 

 bryonic character of the Amnion Animals is the great curva- 

 ture of the head and neck of the embryo. In the Anamnia 

 the embryo is from the first either nearly straight, or else 

 the whole body is bent in a sickle-shaped curve corre- 

 sponding to the curvature of the yelk sac, to which the 

 embryo is attached by its ventral surface ; but there are 

 no marked angles in the longitudinal axis (Plate VI. 

 Fig. F). In all Amnion Animals, on the contrary, the 

 body is very noticeably bent at an early age, so that the 

 back of the embryo is much arched outwards, the head 

 pressed almost at right angles against the breast, and the 

 tail inclined on to the abdomen. The tail extremity, as it 

 bends inwards, approaches so near to the frontal side of 

 the head, that the two often nearly touch (Plates VI, and 

 VII.). This striking triple curvature of the embryonic 

 body, which has already been considered when we studied 

 the Ontogeny of ^lan, and in which we distinguished the 

 skull-curve, neck-curve, and tail-curve (vol. i. p. 871), is a 

 characteristic peculiarity common to the embryos of all 

 Reptiles, Birds, and Mammals. But in the formation of man)» 

 internal organs also, an advance is observable in all the 

 Amnion Animals which ranks them above the highest of 

 the non-aronionate forms. Above all, a partition wall foims 



