336 EMBRYOLOGY. 



SECTION III. 



ZOOLOGICAL IMPORTANCE OF EMBRYOLOGY. 



500. 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 vertebra ta, 

 it may be said, that the organs of the body are successively 

 formed in the order of their organic importance, the most es- 

 sential 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. 



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

 relate to the segmentation of the yolk and formation of the germ, 

 which is a process common to all classes of animals. 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 vertebrata ; 

 an indication afterwards fully confirmed by the successive ap- 

 pearance 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. 



502. 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 

 almost the whole period of embryonic life, the young fish and 

 the young frog scarcely differ at all : 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 farther back in the history of 

 development, we come to a period when no appreciable differ- 

 ence whatever is to be discovered between the embryos of the 

 various departments. The embryo of the snail, when the 



