METAMORPHOSES OP ANIMALS. 359 



566. If, however, we observe the young white-fish just 

 after it has issued from, the egg (fig. 309), the contrast will 

 be less striking. At this period the vertebrae are cartilaginous, 

 like those of the sturgeon ; its mouth also is transverse, and 

 its tail undivided ; at that period the white-fish and the stur- 

 geon are therefore much more alike. But this similarity is 

 only transient; as the white-fish grows, its vertebrse become 

 ossified, and its resemblance to the sturgeon is comparatively 

 slight. As the sturgeon has no such transformation of the 

 vertebrae, and is in some sense arrested in its development, 

 while the white-fish undergoes subsequent transformation, we 

 conclude that, compared with the white-fish, it is really in- 

 ferior in rank. 



567. This relative inferiority and superiority strikes us 

 still more, when we compare with our most perfect fishes 

 (the salmon, the cod &c.) some of those worm-like animals, so 

 different from ordinary fishes that they were formerly placed 

 among the worms. The Amphioxus, represented of its natural 

 size (fig. 375), not only has no bony skeleton, but not even a 

 head, properly speaking. Yet 

 the fact that it possesses a dor- Fig. 375 



sal cord, extending from one ex- 

 tremity of the body to the other, 

 proves that it belongs to the type 

 of the vertebrata (458). But as 

 this peculiar structure is found only at a very early period of 

 embryonic development, in other fishes, we conclude that the 

 Amphioxus holds the very lowest rank in this class. 



568. 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 different 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 metamor- 

 phoses which take place subsequently to birth in others. 



