142 



EXTERNAL FACTORS 



III. 8 



not. All the other elements, however, can only be omitted under 

 penalty of retardation, abnormality, or death (Figs. 74 A and B). 

 The method employed was a simple one. A series of artificial 

 sea-waters was made up, from which, one by one, each of the 

 elements was omitted, another being substituted in its place. 

 Care was taken to make these artificial media approximately 



h 



FIG. 74 A. 



FIG. 74. The necessity of substances contained in sea-water for the 

 normal development of the larvae of sea-urchins. 



a. Without OH. Ciliated stereoblastula of Sphaerechinus. b. KOH 

 has been added, c. Normal blastula of Sphaerechinus. d. Blastula in 

 a K-free medium, e. Reared in K-free and replaced in sea-water (Sphaer- 

 echinus). f. Larva from a medium devoid of Mg (Sphaerechinus). 

 g. Echinus Pluteus with tripartite gut, mouth and coelom sacs, but neither 

 skeleton nor arms ; reared without CaC0 3 or CaS0 4 . h. Normal Pluteus 

 of Echinus, 



isotonic with sea-water, and so exclude a possibly disturbing 

 factor, the alteration of the osmotic pressure. The role of each 

 of these necessary elements or ions will be considered sepa- 

 rately and in some detail. SphaerecMnux and Echinus were the 

 forms principally employed. 



