ITS ZOOLOGICAL IMPORTANCE. 153 



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 various 

 classes of the Animal Kingdom, especially of the verte- 

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

 cessively formed in the order of their organic importance, 

 the most essential being always the earliest to appear. In 

 accordance with 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 preceded 

 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 segmentation of the yolk and the formation of 

 the germ, which is a process common to all classes of ani- 

 mals. It is not until a subsequent period that we trace the 

 dorsal furrow, 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 be- 

 come 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 resemble 

 each other more strongly when examined in the earlier 

 stages of their growth. We have already stated that, during 



