146 EXTERNAL FACTORS III. 8 



Sodium cannot possibly be replaced by lithium, potassium, 

 rubidium or caesium, all of which would at the necessary con- 

 centrations inevitably be poisonous. 



iv. K. 



In the artificial solution employed the small quantity (-07 %) 

 of potassium present in sea-water is simply omitted. 



Without it segmentation except the first few phases is 

 impossible in Echinus. Sphaerechinus, however, segments, but the 

 blastocoel is reduced, the cells are opaque and not vacuolated, and 

 the ova, though ciliated, are motionless and die (Fig. 74 A, d, c}. 



Later stages are also sensitive to the want of potassium. 

 Blastulae gastrulate, but are shrunken, with short archenteron, 

 and in gastrulae the gut does not divide into three. Plutei, like 

 all the others, die when deprived of it. 



In the K-free medium spermatozoa temporarily lose their 

 motility, and such spermatozoa cannot effect fertilization. The 

 fertilization, however, of eggs which have been kept without 

 potassium is possible ; in fact, at this earliest stage, no potassium 

 is absorbed, for eggs fertilized in sea-water develop no further 

 in the K-free solution than do those fertilized and kept con- 

 tinuously in it. 



The absence of potassium in segmentation leaves its effect 

 upon later stages. Two days' exposure is not too long to prevent 

 normal development on removal to sea-water, but five days' 

 exposure causes abnormalities of the skeleton (asymmetrical with 

 several triradiate spicules round the gut) and alimentary canal 

 (no mouth). 



De Vries has shown the importance of potassium for the 

 turgor of young plant-cells, and its function here is probably 

 similar, to promote growth, as the subjoined table of measure- 

 ments shows. Its absence also affects the rate of development 

 (Table XVIII). 



Potassium can in a measure be replaced by rubidium and 

 caesium. The use of lithium either has no effect or, in larger 

 quantities, produces lithium larvae. 



Other forms to which the lack of potassium was found to be 

 fatal were the ova of Asterias and Cotylorhiza, and the adult 



