150 METAMORPHOSES OF ANIMALS. 



speaking. 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 Amphioxus holds the very lowest rank in this class. 



386. Nevertheless, metamorphoses will not indicate the 

 true measure of the perfection of animals, if limited to those 

 changes which take place after birth ; because there are 

 many animals which undergo no changes of great impor- 

 tance after their escape from the egg, and occupy, neverthe- 

 less, a high rank in the Zoological series, as for example, 

 Birds and many Mammals. The question now is, whether 

 such animals are developed according to different plans, 

 or whether their dissimilarity in that respect is merely appa- 

 rent, arising from an incorrect interpretation. To answer 

 this question, let us go back to the period anterior to 

 birth, and see if some connection may not be made out 

 between embryonic changes, and metamorphoses which 

 take place subsequently. 



387. We have already shown that embryonic devel- 

 opment consists in a series of transformations ; the young 

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

 velopment, from what it was in the preceding period. But 

 because these transformations precede birth, and are there- 

 fore not easily observed, does not make them the less 

 important. To be satisfied that these transformations are 

 real metamorphoses, in every respect similar to those 

 which follow birth, we have only to compare, on the other 

 hand, those changes which immediately precede birth with 

 those which immediately 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- 



