46 THE THEORY OF THE GENE 



the sperm. It has not been established, however, that the 

 centrosomes have any specific effects on the develop- 

 mental process. 



From another quarter the significance of the chromo- 

 somes was shown. When two (or more) sperms enter the 

 egg, the three sets of chromosomes that result may be dis- 

 tributed irregularly at the first division of the egg. Four 

 instead of two cells, as in normal development, are 

 formed. It has been shown by a detailed study of such 

 eggs, combined with a study of the development of each 

 of the isolated quarters, that normal development does 

 not take place unless at least one full set of chromosomes 

 is present. At least this is the most reasonable interpreta- 

 tion of the results. Since in these cases the chromosomes 

 are not marked, the evidence does not do more than create 

 a presumption that at least one full set of chromosomes 

 must be present. 



More recently still evidence in favor of such an inter- 

 pretation has come from other sources. It has been shown, 

 for example, that one set of chromosomes alone (haploid) 

 is capable of producing an individual which, to a large 

 extent, is a replica of the normal form, but this evidence 

 also indicates that these haploid individuals are not as 

 vigorous as the normal diploid type of the species. While 

 this difference may depend on factors other than the 

 chromosomes, the presumption remains that two sets of 

 chromosomes are better than one, as things stand. On 

 the other hand, in mosses, where there is a haploid stage 

 in the life cycle, the artificial transformation of the hap- 

 loid stage into a diploid stage does not appear to give an 

 advantage. Furthermore, it remains to be shown that 

 twice the number of chromosomes present in artificial 

 tetraploids confers any advantage over the normal dip- 

 loid set. It is evident, then, that we must be cautious as 

 to the merits of one, two, three, or four sets of chromo- 



