FOUR IMPORTANT THEOREMS 55 



normal number of eighteen chromosomes : such are some of the 

 extraordinary results reached by this clever experimenter. It 

 seems, then, as if fertilisation may, in many cases, be effective 

 without there being any ovum-nucleus present, as if the 

 essential fact were the union of a sperm with a mass of egg- 

 cytoplasm. 



Delage's experiments cited above seem to prove that the 

 nucleus and centrosome of the ovum are not essential to ferti- 

 lisation. Professor Loeb (1899), of Chicago, has made experi- 

 ments which seem to show that the spermatozoon may be 

 dispensed with. In other words, he has been able to induce 

 parthenogenetic development artificially in cases where it does 

 not normally occur. He has been led to believe that the only 

 reason why the eggs of many marine animals do not develop 

 parthenogenetically is that something in the constitution of 

 the sea-water prevents it. This something is the presence or 

 absence of ions of sodium, calcium, potassium, and magnesium, 

 the two former requiring to be reduced, the two latter to be 

 increased. " The mixture of about 50 per cent, ^-n MgCl 2 

 (magnesium chloride) with about 50 per cent, of sea-water was 

 able to bring about the same effect as the entrance of a sperma- 

 tozoon. The unfertilised eggs [of the sea-urchin Arbacia] were 

 left in such a solution for about two hours. When brought back 

 into normal sea-water they began to segment and form blastuke, 

 gastrulae, 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 develop- 

 ment of fertilised eggs. With each experiment a series of control 

 experiments was made to guard against the possible presence 

 of spermatozoa in the sea-water. . . . From these experiments 

 it follows that the unfertilised 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 par- 

 thenogenetically under normal conditions is the constitution of 



