584 STUDIES IN GENERAL PHYSIOLOGY 
In the first solution the eggs reached the thirty-two-cell 
stage. In the second, third, and fourth solutions they formed 
blastule, 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 blastule that had devel- 
oped in normal sea-water continued their motion for two 
days in a solution of 80 c.c. of 42 MgCl, + 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, and CaCl, 
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 réle 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 1°n MgCl, the results 
were not so good. No swimming blastule were formed. In 
solutions of 90 to 80 c.c. of MgCl, with 10 to 70 ce. of 
NaCl a morula stage was reached. 
Mixtures of 19» MgCl, with $n KCl were still less favor- 
able. The solutions with more MgCl, than KCl reached 
the thirty-two-cell stage, or even went a little farther in 
their development. 
Digitized by Microsoft® 
