GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 83 



developmental principle, of fundamental significance, division of labor 

 and the histological differentiation associated with it. In order to 

 understand fully the significance of this principle in development, 

 we must proceed from the thesis that the life of all organic bodies 

 expresses itself in a series of various duties or functions. Organisms 

 take to themselves substances from without ; they incorporate in their 

 bodies that which is serviceable, and eliminate that which is not 

 (function of nutrition and metastasis) ; they can alter the form of 

 their bodies by contraction and extension (function of motion) ; they 

 are capable of reacting upon external stimuli (function of sensibility) ; 

 they possess the ability to bring forth new organisms of their own 

 kind (function of reproduction). In the lowest multicellular organisms 

 each of the individual parts discharges in the same manner as the 

 others the enumerated functions necessary for organic life ; but the 

 more highly an organism is developed, the more do we see that its 

 individual cells differentiate themselves for the duties of life, that 

 some assume the function of nutrition, others that of motion, others 

 that of sensibility, and still others that of reproduction, and that with 

 this division of labor is likewise joined a greater degree of com- 

 pleteness in the execution of the individual functions. The 

 development of a specialised duty likewise leads invariably to an 

 altered appearance of the cell : until the physiological division of 

 labor there always goes hand-in-hand a 'morphological or histological 

 differ en tiation. 



Elementary parts which are especially concerned in the duties of 

 nutrition are distinguished as gland-cells ; again others, which have 

 developed the power of contractility to a greater extent, have 

 become muscle-cells, others nerve-cells, others sexual cells, etc. The 

 cells which are concerned in one and the same duty are for the most 

 part associated in groups, and constitute a special tissue. 



Thus the study of the embryology of an organism embraces chiefly 

 two elements : one is the study of the development of form, the 

 second the study of histological differentiation. We may at the 

 same time add that in the case of the higher organisms the morpho- 

 logical changes are accomplished principally in the earlier stages of 

 development, and that the histological differentiation takes place in 

 the final stages. 



A knowledge of these leading principles will materially facilitate 

 the comprehension of the further processes of development. 



