LARVAE OF ARBACIA PUNCTULATA 



393 



concentrated by evaporation there is, as we have seen, a tend- 

 ency toward increase in the number of spines and other acces- 

 sory processes. This also is typical for NaCl solutions, in 

 which these processes assume slender, spike-like forms. Under 

 both variations of the medium, asymmetry appears in the 

 stronger solutions. The main point of difference between the 

 effects of these two changes of the medium is that in the pure, 

 concentrated sea-water, fusion of the arm-rods occurs rather 



early, whereas in the NaCl solutions, a retardation of the process 

 is evident. 



The specimens in the cultures made hypertonic by the addi- 

 tion of NaCl show the modifications common to the two methods 

 discussed above, — that is, prominent processes and slight irregu- 

 larities of structure. In respect to fusion of the arm-rods, 

 they agree more closely with the specimens in the solutions in 

 which sea-water was diluted with iso-tonic NaCl, in that a 

 retardation of this process takes place. It is evident, then, that 



