78 RALPH S. LILLIE. 



Such a conclusion directs our particular attention to the general 

 nature of the conditions controlling growth. 



Growth and development may be controlled to a greater or 

 less degree by various artificial means ; and much of experimental 

 embryology is concerned with modifying the rate and character 

 of either process. In this way many definite hereditary char- 

 acters may be profoundly altered, or in some instances their 

 appearance may be altogether suppressed. A simple and in- 

 structive instance is described by Loeb. 1 The sea-urchin egg 

 will develop to the gastrula stage in a balanced solution of sodium, 

 potassium, and calcium chloride; if in addition to these salts 

 some sodium carbonate is present, the skeletal spicules may form 

 and a pluteus larva develop, but not otherwise. The skeleton is 

 an inherited character; its formation, however, is dependent 

 upon the presence of sodium carbonate in the surrounding 

 medium, as well as upon the organization of the germ; the neces- 

 sary carbonate must be furnished to the germ from without, or 

 the specific formative process is unable to take place. Such a 

 result is not difficult to understand. Development, like growth, 

 is a matter of metabolism, and primarily of constructive meta- 

 bolism; hence it is influenced by any condition that influences 

 metabolism; accordingly the presence or absence of food, oxygen, 

 water, salts, vitamines, hormones, as well as the conditions of 

 temperature, may each and all have determinative relations to 

 the total process. It is significant, however, that the specific 

 characters of the organism, those which, according to the present 

 view, express the chemical specificity of its structural proteins, 

 seem never to be essentially altered by such changes of condition, 

 although their appearance may be prevented or the degree of 

 their development modified. Whatever structural characters 

 appear in development are those characteristic of the species; this 

 statement may be qualified to the degree required to take into 

 consideration the facts of mutation (these suggest that under 

 exceptional conditions new r structural proteins may be synthe- 

 sized) ; but the essential fact which w r e wish to express is the 

 tenacity with which the organism preserves its specificity. At 

 least this specificity can be modified, if at all, only by gradual 



1 J. Loeb, Amer. Journ. Physiol., 1900, Vol. 3, p. 441. 



