2^5 BIOLOGICAL LECTURES. 



contributed by Dzierzon, who discovered that the unfertihzed 

 eo-o-s of bees develop and give rise to males, while the fertilized 

 eggs give rise to females. Similar conditions seem to exist in 

 wasps. It is, moreover, certain that a few crustaceans show 

 parthenogenesis. 



A beginning of parthenogenetic development had been 

 observed in the case of a great many marine animals which 

 develop outside of the female in sea water. It was found that 

 such eggs when left long enough in sea water may develop into 

 two or three cells, but no further. (2) On the other hand, in 

 ovaries of mammals now and then eggs were found that were 

 seg-mented into a small number of cells. These facts and the 

 occurrence of a certain class of tumors in the ovary, the so-called 

 teratomata, suggest the possibility of at least partial partheno- 

 genesis in the eggs of mammals. But all these phenomena 

 were considered to be of a pathological character. It must be, 

 however, admitted that we cannot utilize these facts with any 

 degree of certainty for the theory of fertilization, as in this 

 case certainty can only be obtained by the experiment. It 

 was not until very recently that such experiments were made. 



II. 



Eight years ago I observed that if the fertilized eggs of the 

 sea urchin were put into sea water whose concentration was 

 raised by the addition of some neutral salt they were not able 

 to segment, but that the same eggs, when put back after they 

 had been in such sea water for about two hours, broke up into 

 a large number of cells at once instead of dividing successively 

 into two, four, eight, sixteen cells, etc. Of course it is neces- 

 sary for this experiment that the right increase in the concen- 

 tration of the sea water be selected. (3) The explanation of 

 this fact is as follows : The concentrated sea water brings about 

 a change in the condition of the nucleus which permits a division 

 and a scattering of the chromosomes in the egg. As soon as 

 the egg is put back into normal sea water it breaks up into as 

 many cleavage cells at once as nuclei or distinct chromatin 

 masses had been preformed in the egg. Morgan tried the same 



