INTRODUCTION 15 



must remember that we do so for the sake of memory and 

 of precision ; we must by no means shut our eyes to the fact 

 that the relations of one system to another, as we discover them 

 in the course of dissection, are of at least as much importance 

 as the plans of those systems considered apart from one anothei- 

 and the rest of the body. Our object, then, is twofold : to get 

 an accurate idea of each system of organs considered by itself, 

 and to form a conception of the manner in which those 

 systems are united together to make up the sum-total of the 

 organisation of the animal in question. 



It is perhaps the most convenient plan to consider the 

 different systems of organs in the order in which they make 

 their appearance in the course of development, or, to speak 

 more accurately, according to the germinal layer from which 

 they are developed, for by so doing we shall bring together the 

 results of anatomical and embryological study, and make the 

 one support and illustrate the other. But, as the study of the 

 adult anatomy of a vertebrate type must precede that of its 

 development, we shall begin by describing the organs of the 

 frog in the order in which they are most conveniently studied 

 in the course of dissection. 



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