DEVELOPMENT. 89 



to that already existing, but exceeding in quantity that which 

 is being destroyed. In the process of growth, therefore, in 

 the strict sense of this term, there cannot occur any change 

 in the actual form or composition of the growing body. The 

 part, or the organism as a whole, increases in density or 

 size by the addition of particles similar to those of which it 

 already consists ; but no change takes place in its essential 

 characters, or in the functions which it is capable of dis- 

 charging. 



DEVELOPMENT. 



We have in the preceding been considering the processes 

 by which an organism, or any part of an organism, is enabled 

 to grow, or, when fully grown, is enabled to maintain itself 

 for a longer or shorter period in a stationary condition. We 

 have now very briefly to consider the processes by which 

 any organism becomes what we see it to be, or by which a 

 given organ is for the first time formed and brought to 

 maturity. To all these processes the term " Development " 

 is applied, but w^e are here only concerned with those which 

 relate to the organism as a whole. 



From this point of view the term Dcvclopviait includes all 

 those changes which a germ undergoes before it assumes 

 the characters of the perfect individual ; and the chief dif- 

 ferences which are observed in the process as it occurs in 

 different animals consist simply in the extent to which these 

 changes are external and visible, or are more or less com- 

 pletely concealed from view. For these differences the 

 terms "transformation" and "metamorphosis" arc em- 

 ployed ; but they must be regarded as essentially notiiing 

 more than variations of development. 



Transformation is the term employed by Quatrefages to 

 designate " the series of changes which every germ under- 

 goes in reaching the embrj'onic condition ; those which we 

 observe in every creature still within the e^g ; those, finally, 



