Chap. I.] INTRODUCTORY. 



Now all those problems which deal with form and structure 



^re called morphological problems ; and Morphology, which deals 



with these problems, forms a well-marked department of Animal 



Biology. From what has just been said, moreover, it follows 



that this department comprises three divisions : 



1. The morphology of organisms (zoology). 



2. The morphology of organs (anatomy). 



3. The morphology of tissues (histology). 



But the form and structure of the frog does not remain the 

 same in all periods of its existence. Beginning life as an egg, 

 it is hatched as a tadpole, and only reaches the full stature of 

 froghood after several metamorphoses. There is, therefore, a 

 Development of the organism as a whole, of the organs within 

 the organism, -and of the tissues of which the organs are com- 

 posed t Hence there arises a second and most important set of 

 morphological problems of development, which may be divided 

 into 



1. The development of organisms j f f ** "^nal (ontogeny). 



( b. of the race (phytogeny). 



2. The development of organs (organology) ) embryolo gy. 



3. The development of tissues (histogenesis) ) 



So far we have only considered the problems that arise out of 

 a study of form or structure. We have regarded the frog 

 merely as a piece of mechanism. But every structure has its 

 special office. Every organ in the frog's body has its particular 

 work to do, its function as it is called. The muscles contract or 

 shorten, and so produce motion, the heart beats and ministers 

 to the circulation of the blood, the lungs are for respiration, the 

 eye for seeing, the glands for secreting, and so forth. In these 

 various organs, moreover, special tissues perform the special part 

 of the work, and within these tissues special cells are developed. 

 Finally, all these functions are to be made subservient to the 

 general good of an organism which is set in the midst of more 

 or less complex surrounding conditions. And then the frog 

 himself, as one organism among many, has to perform his 

 special function in the economy of Nature. Hence arises a new 



