MAN AS AN AMNION ANIMAL. 137 



within the simple ventricle of the heart, dividing it into a 

 right and a left ventricle. In connection with the complete 

 metamorphosis of the gill-arches, a further development of 

 the organ of hearing takes place. A considerable advance 

 is also noticeable in the development of the brain, the skele- 

 ton, the muscular system, and other parts. Finally, the 

 reconstruction of the kidneys must be regarded as a most 

 important modification. In all the lower Vertebrates as yet 

 considered, we have found the primitive kidneys, which 

 appear very early in the embryos of all higher Vertebrates 

 up to Man, acting as a secretory or urinary apparatus. In 

 Amnion Animals, however, these early primitive kidneys 

 lose their function at an early period of embryonic life, and 

 it is assumed by the permanent " secondary kidneys," which 

 grow out of the terminal portion of the primitive kidney 

 ducts. 



Looking back at the whole of these characters of Amnion 

 Animals, it is impossible to doub" that all animals of this 

 group, all Reptiles, Birds, and Mammals, had a common 

 origin, and constitute a single main division of kindred 

 forms. To this division belongs our own race. In his 

 whole organization and germ-history Man is a true Amnion 

 Animal, and, in common with all other Amniota, has 

 descended from the Protamnion. Although this whole 

 group originated at the end, or perhaps even in the middle, 

 of the Palaeozoic Epoch, it did not attain its full de- 

 velopment and its full perfection till the Mesozoic Epoch. 

 The two classes of Birds and Mammals then first appeared. 

 Nor did the Reptilian class develop in its full variety 

 until the Mesozoic Epoch, which is, therefore, called the "Age 

 of Reptiles." The unknown and extinct Protamnion, the 



