ARTIFICIAL PRopucTIOoN oF NormMaL Larva 589 
a small amount of Na,CO, allowed the formation of a skel- 
eton. The addition of Na,CO, causes an addition of HO 
ions as well as of CO, ions. Which of the two are respon- 
sible for the formation of a skeleton? The substitution of 
KHO for the Na,CO, did not allow the formation of a 
skeleton. We must therefore conclude that it is the CO, 
ton which is essential. 
Conclusions.—We thus see that a mixture of 96 c.c. £n 
NaCl (or NaBr) + 2 cc. $n KCl + 2 cc, 127 CaCl is suffi- 
cient to allow the fertilized egg of Arbacia to develop into 
the gastrula stage. But does this force us to conclude that 
the three kations Na, K, and Ca are utilized by the egg for 
the process of development? I think that our previous ex- 
periments on Fundulus’ may serve as a criterion in answer- 
ing the question. In a pure NaCl solution the young fish 
died rapidly, while in the above-mentioned mixture they re- 
mained alive. And yet this same fish could live indefinitely 
in distilled water. This proves that it does not require any 
ions from the surrounding medium. It might be possible 
that only Na ions were needed for the development of the 
sea-urchin egg. In this case the K and Ca ions would have 
to be present in order to prevent the poisonous effects which 
a pure NaCl solution would produce. On the other hand, 
we found that ina mixture of MgCl, and CaCl, which is 
practically free from Na ions the eggs can develop and al- 
most reach the blastula stage. This makes it still more dif- 
ficult to state positively that the Na ions of the surrounding 
medium are needed for the development of the sea-urchin 
egg. Perhaps it is safest to assume that for the process of 
cell-division and development a certain physical condition— 
a certain labile equilibrium—of the protoplasm of the col- 
1For further facts concerning the formation of a skeleton in sea-urchins see 
Hersst, Archiv fiir Entwickelungsmechanik, Vol. II (1896), p. 455; and DRIEscH. 
ibid., Vol. IX (1899), p. 137. 
2 Part II, p. 544. 
Digitized by Microsoft® 
