182 METAMORPHOSES OF ANIMALS. 



has no bony skeleton, but not even a head, properly speak- 

 ing. Yet the fact that it possesses a dorsal cord, extending 

 from one extremity of the body to the other, proves that it 

 belongs to the type of Vertebrates. But as this peculiar 

 structure is found only at a very early period of embryonic 

 development, in other fishes, we conclude that the Amphi- 

 oxus holds the very lowest rank in this class. 



386. Nevertheless, the metamorphoses of animals after 

 birth, will, in many instances, present but trifling modifica- 

 tions of the relative rank of animals, compared with those 

 which may be derived from the study of changes previous 

 to that period, as there are many animals which undergo no 

 changes of great importance after their escape from the egg, 

 and occupy, nevertheless, a high rank in the Zoological 

 series, as, for example, Birds and Mammals. The question 

 is, whether such animals are developed according to differ- 

 ent plans, or whether their peculiarity in that respect is 

 merely apparent. To answer this question, let us go back 

 to the period anterior to birth, and see if some parallel may 

 not be made out between the embryonic changes of these 

 animals and the metamorphoses which take place subse- 

 quently to birth in others. 



387. We have already shown that embryonic develop- 

 ment consists in a series of transformations ; the young ani- 

 mal enclosed in the egg differing at each period of its de- 

 velopment, from what it was before. But because these 

 transformations precede birth, and are, therefore, not generally 

 observed, they are not less important. To be satisfied that 

 these transformations are in every respect similar to those 

 which follow birth, we have only to compare the changes 

 which immediately precede birth with those which immediate- 

 ly follow it, and we shall readily perceive that the latter are 

 simply a continuation of the former, till all are completed. 



388. Let us recur to the development of fishes for illus- 



