III. 8 CHEMICAL COMPOSITION 147 



Amp/iioxus. Potassium is also necessary for the contractions of 

 muscles (umbrella and tentacles of Obilia). 



TABLE XVIII 



Showing the effect of potassium upon the growth of 

 Sea-urchin blastulae. (After Herbst.) 



Ratio (unit - jl^ mm.) of cross- 

 diameter to long diameter. 



After Without K. With K. 



18-5 204 



18 hours 

 24 hours 

 45 hours 



18-7 20-7 



184 2L9 



19-1 22-7 



19-7 27-5 



191) 27-8 



Showing the effect of potassium on the rate of development of 

 Sea-urchin larvae. (After Herbst.) 

 Artificial sea-water with 

 After .008 % KC1. .016 % KC1. -024 % KC1. 



36 hours Small gastrulae. Nearly Plutei. Plutei. 



60 hours No mouth ; gut not Small Plutei. Fully formed Plutei. 

 constricted. 



v. Mg. 



The fertilized ova were placed in a solution which did not 

 include the -32 % MgCl 2 and the -26 % MgSO 4 present in sea- 

 water. 



Segmentation proceeds normally, but the blastula is slightly 

 smaller than when magnesium is present (the ratio of the 

 diameters is f ). The skeleton, however, though bilaterally laid 

 down, is retarded and deformed (magnesium is present in the 

 skeleton of the sea-urchin and possibly in that of the Pluteus) 

 and the gut is not properly differentiated, not tripartite and 

 without a mouth (Fig. 74 A,/). 



When the MgSO 4 is replaced by an isotonic quantity of 

 Xa 2 SO 4 the results are the same. 



In the Mg-free solution cilia cease beating, development is 

 retarded (Table XIX), and, though the spermatozoa retain their 

 motility, the ova are so injured that fertilization is impossible 

 unless they are restored to sea-water. The ova, in fact, seem 

 to have a store of Mg which they lose in the Mg-free mixture. 

 Fertilization can, however, take place without magnesium if the 



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