26 REMARKS ON THE PRINCIPLES OF 



anatomist miglit exaggerate their importance ; yet anatomical characters 

 deserve the most careful study, and in innumerable instances furnish, 

 the most raluable tests where external form or habit might deceive 

 us. So embryology is one of our best guides in determining funda- 

 mental differences of structure, though some of the differences it 

 brings under our notice are secondary, and must not be made too 

 much of. Let us duly weigh the embryological and anatomical diffe= 

 rences between Reptilia and Amphibia, in, respect to their number 

 and value, and we may perhaps see our way to a satisfactory conclu- 

 sion. In Amphibians the eggs, as in Fishes, are excluded before 

 impregnation, and are always destitute of any hard protecting cover- 

 ing. There is also, at the commencement of developement, the same 

 partial segmentation of the yolk as in fishes. Again, the alantois and 

 amnion are wanting in Amphibians as in Fishes, present in Reptilia as 

 in Birds and Mammals. In Amphibia, progressive changes which in 

 Reptilia, as in higher animals, take place within the egg, occur after 

 the exclusion of the young animal, constituting its metamorphoses 

 which are specially manifested in the higher members of the group. 

 These seem sufficiently striking and important embryonic characters. 

 Turning to those which belong to the matured structure, we notice 

 the naked skin distinguishing Amphibia equally from Fishes and Rep- 

 tiles, and to which the only exception perhaps is Lepido siren, if we 

 may assume, in opposition to the high authority of Owen, that on the 

 whole Amphibian characters prevail in it over the Piscine. In 

 Amphibia the two modes of aeration by lungs and gills, the former of 

 which is suppressed in Fishes, the latter in Reptilia and the higher 

 animals, co-exist either during the whole of life or at least in its earlier 

 stages. In Amphibia the ribs which are highly developed both in 

 Fishes and Reptiles are absent or only rudimentary, and the connec- 

 tion of the bones of the head with the atlas is by two articular tuber- 

 cles, instead of one as in Reptilia and Birds, a structure apparently con- 

 nected with the imperfect development of the bones of the skull in 

 Amphibia. "Without seeking other characters, these seem sufficient 

 to show that Amphibia occupy a well marked intermediate position 

 between Reptilia and Fishes, and it may be doubted whether most of 

 the classes of the animal kingdom have more striking and decisive dis- 

 tinctions, though the long practice of naturalists in combining them 

 with Reptilia and the near resemblance of the analogous forms are 

 apt to confuse our ideas on the subject. The circumstance that 



