viii PREFACE 



The causes which determine the course of an organism's 

 development are numerous, but fall into two general categories ; 

 namely, internal causes, which include the qualities of the devel- 

 oping protoplasm ; and external causes, which include the 

 chemical and physical properties of the environment in which 

 the protoplasm is developing. The internal and external causes 

 may be studied separately, and in order to disentangle their 

 effects they must needs be studied separately. It is the pur- 

 pose of the present work to consider the effects resulting from 

 external causes. 



When we wish to isolate the separate effects in any complex 

 of causes, we must resort to the well-known procedure of 

 experimentation, and we find, indeed, that these external 

 causes lend themselves readily to this method of treatment. 

 Accordingly we call in experiment to get an insight into the 

 causes of organic form, and thus justify the name which we 

 have applied to our study, Experimental Morphology. 



The primary subdivision of the subject is based upon the 

 morphogenic processes to be treated of ; and of these, four 

 principal classes may be recognized. The first includes those 

 processes which are characteristic of all living protoplasm; 

 the second, those connected with growth ; the third, those 

 involved in cell-division ; and the fourth, those producing 

 differentiation. It is proposed to devote one part of the work 

 to each of these four classes of processes. 



The secondary subdivision may be based upon the chemical 

 and physical agents whose effects we wish to isolate. These 

 may be grouped into eight categories, determined largely by 

 convenience ; namely, 1, chemical substances ; 2, water ; 3, den- 

 sity of the medium ; 4, molar agents ; 5, gravity ; 6, electri- 

 city ; 7, light ; and 8, heat. It is proposed to devote one 

 chapter to a consideration of the effects of each of these agents 

 upon protoplasm, upon growth, upon cell-division, and upon 

 differentiation. 



Two words should be said about the point of view from 

 Avhich this book has been written. In the first place, the 

 developing organism is regarded as a living organism, and as 

 such endowed with irritability and capacity of response ; con- 

 sequently, at the outset, we must especially consider the phe- 



