GENERAL CHARACTERS OF LIVING ORGANISMS 39 



of development in any particular species requires 

 constancy in the external conditions. For example, the 

 developing sea-urchin larva forms a skeleton of a charac- 

 teristic and often complex design in sea water and in 

 artificially balanced media containing the chief salts 

 of sea water together with some sodium carbonate; 

 the formation of this skeleton causes the larva to assume 

 the triangular and long-armed shape characteristic of 

 the pluteus stage. But if the carbonate is omitted 

 from the medium, the skeleton fails to form, and 

 development does not proceed beyond the gastrula stage.^ 

 The special form of the skeleton is said to be "inherited"; 

 this experiment shows, however, that it is dependent on 

 the presence of carbonate quite as «iuch as on the 

 presence of special determinants in the germ. 



Such an example shows further that constancy in 

 the normal sequence of growth processes is the essential 

 condition for the manifestation of heredity; it also 

 illustrates the composite nature of the physiological 

 factors determining the production of any adult form- 

 character; in all cases the co-operation of definite 

 "internal" and "external" factors is necessary to yield 

 the final result. Many cases are also known where 

 development is altered in a definite manner by the addi- 

 tion of special growth-modifying substances; a well- 

 known example of such influence^ exerted by a simple 

 inorganic substance, is the production of cyclopia in 

 fishes by increasing the magnesium content of sea 

 water ;^ other substances and conditions (alcohol 



^ J. Loeb, American Journal of Physiology, III (1900), 441. 



2 Stockard, Arch. Entwickl. Organ., XXIII (1907), 249; Journal of 

 Experimental Zoology, VI (1909), 285. 



