ZOOLOGICAL IMPORTANCE OF EMBRYOLOGY. 337 



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 ; but 

 the class and the group are not yet indicated. 



\t'ter this account of the history of the develop- 

 ment of the egg, the importance of embryology to the study 

 of zoology cannot be questioned. For evidently, if the for- 

 mation 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 classification. 

 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 ver- 

 tebrata, the articulata, the mollusca, and the radiata, cor- 

 responds perfectly with the gradations displayed by embry- 

 ology. 



504. This classification, as has been already shown, is 

 founded essentially on the organs of animal life, the nervous 

 m and the parts belonging thereto, as found in the per- 

 fect animal. ^"ow, 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 vege- 

 tative 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 afterwards. There- 

 fore a classification, to be true and natural, must accord with 

 the succession of organs in the embryonic development. This 

 coincidence, while it corroborates the anatomical principles of 

 Cuvier's classification of the animal kingdom, furnishes us 

 with new proof that there is a general plan displayed in 

 every kind of development. 



505. Combining these two points of view, that of Embry- 

 ology and that of Anatomy, the four divisions of the animal 

 kingdom may be represented by the four figures which are to 

 be found, at the centre of the diagram, at the beginning of 

 the volume. 



500'. The type of VERTEBRATA, having two cavities, one 

 above the other, the former destined to receive the nervous 

 system, and-.the latter, which is of a larger size, for the intes- 

 tines, is represented by a double crescent united at the centre, 

 and closing above, as well as below 



