542 STUDIES IN GENERAL PHYSIOLOGY 
MgCl, with about 50 per cent. of sea-water was able to 
bring about the same effect as the entrance of a sper- 
matozoon.' The unfertilized eggs were left in such a solu- 
tion for about two hours. When brought back into normal 
sea-water they began to segment and form blastule, gastrula, 
and plutei, which were normal in every respect. The only 
difference was that fewer eggs developed, and that- their 
development was slower than in the case of the normal 
development of fertilized eggs. With each experiment a 
series of control experiments was made to guard against the 
possible presence of spermatozoa in the sea-water. Un- 
fertilized eggs of the same female were brought into normal 
sea-water, and in solutions with too little MgCl,. Neither 
in the normal sea-water nor in any of these solutions with 
too little MgCl, did one single egg develop into a blastula 
or show anything more than the beginning of a segmenta- 
tion after a long time. 
4, From these experiments it follows that the unfertilized 
egg of the sea-urchin contains all the essential elements for 
the production of a perfect pluteus. The only reason that 
prevents the sea-urchin from developing parthenogenetically 
under normal conditions is the constitution of the sea-water. 
The latter either lacks the presence of a sufficient amount of 
the ions that are necessary for the mechanics of cell-division 
(Mg, K, HO, or others), or it contains too large a quantity 
of ions that are unfavorable to this process (Ca, Na, or 
others), or both. All the spermatozoon needs to carry into 
the egg for the process of fertilization are ions to supple- 
ment the lack of the one or counteract the effects of the 
other class of ions in the sea-water, or both. The sper- 
matozoon may, however, carry in addition a number of 
1 The subsequent experiments proved that the increase in the concentration of 
the sea-water caused the development of the eggs. I was aware of this possibility 
and was looking for it, but was misled through an error in the preparation of the 
solutions (which I had intrusted to others). This error was afterward discovered 
and corrected. [1903] 
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