154 EMBRYOLOGY. 



almost the whole period of embryonic life, the young fish 

 and the young frog scarcely differ at all, (313 :) so it is also 

 with the young snake compared with the embryo bird. The 

 embryo of the crab, again, is scarcely to be distinguished 

 from that of the insect ; and if we go still further back in 

 the history of development, we come to a period when no 

 appreciable difference whatever is to be discovered between 

 the embryos of the various departments. The embryo of 

 the snail, when the germ begins to show itself, is nearly the 

 same as that of a fish or a crab. All that can be predicted 

 at this period is, that the germ which is unfolding itself 

 will become an animal ; the class and the group are not yet 

 indicated. 



321. After this account of the history of the development 

 of the egg, the importance of Embryology to the study of 

 systematic Zoology cannot be questioned. For evidently, if 

 the formation of the organs in the embryo takes place in an 

 order corresponding to their importance, this succession must 

 of itself furnish a criterion of their relative value in classifi- 

 cation. Thus, those peculiarities that first appear should be 

 considered of higher value than those that appear later. In 

 this respect, the division of the Animal Kingdom into four 

 types, the Vertebrates, the Articulates, the Mollusks, and the 

 Radiates, corresponds perfectly with the gradations displayed 

 by Embryology. 



322. This classification, as has been already shown, (61,) 

 is founded essentially on the organs of animal life, the ner- 

 vous system and the parts belonging thereto, as found in the 

 perfect animal. Now, it results from the above account, 

 that in most animals the organs of animal life are precisely 

 those that are earliest formed in the embryo ; whereas those 

 of vegetative life, on which is founded the division into 

 classes, orders, and families, such as the heart, the respiratory 

 apparatus, and the jaws, are not distinctly formed until after- 



