xii INTRODUCTION 



vantage of giving the student closely allied but different examples to 

 work out with these descriptions. 



The work is primarily a treatise on animal histology. Yet some of it 

 is due to researches that have been carried out on plant tissues. In 

 several places fundamental facts are illustrated by materials taken from 

 the vegetable kingdom. 



The arrangement and order of presentation of the matter have been 

 given much thought. It has been suggested by many that the arrange- 

 ment have some relation to the onfogenetic development of the animal 

 body and that the classification of the tissues be based upon their origin 

 from the dividing oosperm. While the many advantages of such a treat- 

 ment have been fully realized, the writers have felt that a really fairer 

 and more logical method would be an arrangement on a basis of function 

 in the first place with ontogenetic and a possible phylogenetic origin 

 treated of necessity in the second place. We believe that such a direct 

 treatment is not only more convenient and clear, but that it is more logi- 

 cal and true, and that it will serve the better to correlate the student's 

 conceptions concerning homology and analogy and kindred relationships. 



Perhaps the principal practical objection to the use of the embryo- 

 logical idea in the arrangement of this volume would be the great differ- 

 ences that exist in the fundamental facts of cell-lineage and histogenesis 

 among the principal groups of animals. So long as the work dealt with 

 the vertebrates alone, this difficulty would not emerge, but in a really 

 broad treatment on this basis of almost any principal tissue, much use- 

 less repetition and lack of unity would be encountered. The great value 

 of treating the subject from the view point of function can better be under- 

 stood when it is remembered that all structures exist only for the purpose 

 of performing certain functions in some particular way. 



The embryological method has not been slighted, however, in arrang- 

 ing this course. It either parallels, or even supplants, at a number of 

 points, the arrangement we have adopted, and it is given a fair if second- 

 ary exposition throughout the work. This, as well as other arrangement, 

 has necessitated some small repetition of certain ideas. The writers have 

 not hesitated to repeat leading ideas when it was needful and possible, at 

 the same time, to look at them from different points of view. 



A full bibliography has not been inserted in consideration of the fact 

 that the book is intended for college students, and it was thought that 

 a long list of articles, some of them old and others inaccessible, would 

 tend to discourage any further reading. From one to three broad and 

 modern articles by recognized authorities have been mentioned after 

 each part, together with the best general reference books. If the student 

 is to go farther into the subject, the instructor should accustom him to 

 looking up references in the " Zoologischer Jahresbericht" of Naples, 



