ARTIFICIAL PRODUCTION OF NORMAL LARV^I 589 



a small amount of Na 2 CO 3 allowed the formation of a skel- 

 eton. The addition of Na 2 CO 3 causes an addition of HO 

 ions as well as of CO 3 ions. Which of the two are respon- 

 sible for the formation of a skeleton ? The substitution of 

 KHO for the Na 2 CO 3 did not allow the formation of a 

 skeleton. We must therefore conclude that it is the CO 3 

 ion which is essential. 1 . 



Conclusions. We thus see that a mixture of 96 c.c. ^n 

 NaCl (or NaBr) + 2 c.c. f n KC1 + 2 c.c. y n 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 2 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 in a mixture of MgCl 2 and CaCl 2 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- 



iFor further facts concerning the formation of a skeleton in sea-urchins see 

 HEEBST, Archiv filr Entwickelungsmechanik, Vol. II (1896), p. 455; and DRIESCH. 

 ibid., Vol. IX (1899), p. 137. 



2 Part II, p. 544. 



