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 much as on the 

 presence of special determinants in the germ. 



Such an example show^s 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, 111 (1900), 44i- 

 ^Stockard, Arch. Eulwkkl. Organ., XXTTT (1907), 249; Jourual of 

 Experimental Zoology, VI (1909), 285. 



