INDIVIDUAL DEVELOPMENT 247 



development, but useless as an example of a formative stimulus. 

 Morphogenetic behavior, like behavior in other fields, is not a function 

 of the stimulus as to its specificity, but it is prescribed and limited 

 by the reaction capacity of the system. One example is as good as 

 many. We shall not find this principle contradicted by any of the 

 known data of the physiology of development. 2 



Metabolic Relations. — When we consider to what an extent the 

 nature of every biological character is given in its chemical composi- 

 tion, it can be readily understood that, to some authors, physiological 

 chemistry should seem the complete basis of heredity. The characters 

 of every tissue of tbe body are absolutely dependent on their chemical 

 composition, and even slight variations in chemical composition may 

 completely alter function, appearance or form. For such a statement 

 examples are entirely unnecessary. 



The development of characters in the individual is dependent upon 

 the occurrence of definite chemical reactions, upon their rate and upon 

 their degree of completion. It has been shown that the law of accelera- 

 tion of embryonic development in correspondence with rise of tempera- 

 ture is the same in principle as the law of acceleration of chemical reac- 

 tions by temperature increase. Numerous experiments have been made 

 on the character of development in the absence of one or other or 

 combinations of the elements normal to protoplasm, with the aim of 

 determining their role in development. Herbst, for instance, has made 

 a series of experiments on the development of larvae of the sea urchin 

 in artificial sea waters, in the composition of which definite elements 

 are wanting. He shows, for instance, that in sea water made up 

 without calcium the skeleton fails to develop, and that the form of 

 the larva resulting is profoundly modified from the normal. In other 

 experiments the potassium or sulphur, or iron, etc., is omitted from 

 the solution, and the effect on the development noted. Other experi- 

 menters have maintained that the presence of specific chemical elements 

 in excess has definite morphological consequences. 



As regards complex substances and their role in morphogenesis, but 

 little is actually known. Eecent results indicate that the egg con- 

 tains substances of complex chemical composition which are essential 

 for the development of specific parts or tissues. Thus certain experi- 

 ments consist in the removal of definite parts of the egg containing 

 specific materials; and in the subsequent development specific parts of 

 the embryo are wanting. In other experiments, by Conklin, the trans- 

 ference of definite substances from their normal location by means of 

 centrifugal ' force is followed by the development of corresponding 



3 Stimuli in the sense in which we use the word involve merely the im- 

 pinging of energies on the stimulated system; if substantive additions are 

 involved we have more than a mere stimulus, to the extent that substances are 

 added to the system. 



