GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 83 



developmental principle of fundamental significance, division of labor 

 and the histological differentiation associated ivith 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 

 dividual cells differentiate themselves for the duties of life, that 

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

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

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

 eteness in the execution of the individual functions. The 

 evelopment of a specialised duty likewise leads invariably to an 

 altered appearance of the cell : with the physiological division of 

 bor there ahuays goes hand-in-hand a morphological or histological 

 ifferentiation, 



Elementary parts which are especially concerned in the duties of 

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

 (veloped the power of contractility to a greater extent, have 

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

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

 rt associated in groups, and constitute a special tissue. 

 Thus the study of the embryology of an organism embraces chiefly 

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

 cond the study of histological differentiation. We may at the 

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

 ical changes are accomplished principally in the earlier stages of 

 velopmeiit, and that the histological differentiation takes place in 

 e final stages. 



A knowledge of these leading principles will materially facilitate 

 e comprehension of the further processes of development. 



