122 EMBRYOLOGY. 



SECTION III. 



ZOOLOGICAL IMPORTANCE OF EMBRYOLOGY. 



318. As a general result of the observations which have 

 been made, up to this time, on the embryology of the vari- 

 ous classes of the Animal Kingdom, especially of the verte- 

 brates, it may be said, that the organs of the body are succes- 

 sively formed in the order of their organic importance, 

 the most essential being always the earliest to appear. In 

 consequence of this law, the organs of vegetative life, the 

 intestines and their appurtenances, make their appearance 

 subsequently to those of animal life, such as the nervous 

 system, the skeleton, &c. ; and these, in turn, are pre- 

 ceded by the more general phenomena belonging to the 

 animal as such. 



319. Thus we have seen that, in the fish, the first changes 

 relate to the formation and furrowing of the germ, W 7 hich is 

 a character common to all classes of animals. It is not un- 

 til a subsequent period that we trace the dorsal groove, 

 which indicates that the forming animal will have a 

 double cavity, and consequently belong to the division of 

 the vertebrates ; an indication afterwards fully confirmed 

 by the successive appearance of the brain and the organs of 

 sense. Later still, the intestine is formed, the limbs 

 become evident, and the organs of respiration acquire 

 their definite form, thus enabling us to distinguish with 

 certainty the class to which the animal belongs. Finally, 

 after the egg is hatched, the peculiarities of the teeth, 

 and the shape of the extremities mark the genus and 

 species. 



320. Hence, the embryos of different animals resem- 



