584 STUDIES IN GENEEAL PHYSIOLOGY 



In the first solution the eggs reached the thirty-two-cell 

 stage. In the second, third, and fourth solutions they formed 

 blastulae, which, however, did not move. I first thought 

 that for the motility of the cilia the presence of other ions 

 might be required, but I found that blastulse that had devel- 

 oped in normal sea-water continued their motion for two 

 days in a solution of 80 c.c. of l -g n MgCl 2 +20 c.c. of sea- 

 water. It is possible, however, that in such a solution cilia 

 cannot be formed. I placed a lot of these eggs that had 

 reached the blastula stage in a mixture of MgCl 2 and CaCl 2 

 in normal sea-water. The next morning they moved about 

 in the most lively manner. It is certainly contrary to the 

 current ideas concerning adaptation that the egg of Arbacia 

 should reach the blastula stage in a solution which is practi- 

 cally free from Na ions. 



In the fifth solution only very few eggs segmented 

 and reached the eight-cell stage, while the other solutions 

 were still worse. The segmentation was more regular the 

 more Mg the solution contained, and became more irregular 

 the more the Ca ions predominated. One of the chief features 

 of this irregularity was the unequal size of the cleavage 

 cells. As in certain eggs the unequal size of the cleavage 

 cells is a characteristic feature which plays a great r6le in 

 the theories of development, it is of interest that such differ- 

 ences can be brought about through the presence of a cer- 

 tain quantity of definite ions, especially of Ca and Na ions. 



In the mixtures of -|n NaCl with ^ n MgCl 2 the results 

 were not so good. No swimming blastula3 were formed. In 

 solutions of 90 to 30 c.c. of MgCl 2 with 10 to 70 c.c. of 

 NaCl a morula stage was reached. 



Mixtures of y n MgCl 2 with -|w KC1 were still less favor- 

 able. The solutions with more MgCl 2 than KC1 reached 

 the thirty-two-cell stage, or even went a little farther in 

 their development. 



