No. 524] CHROMOSOMES AND HEREDITY 459 



somes than in the four-fold type — hence he argues the 

 greater frequency of normal development. 



Boveri 's chief results, however, were obtained by iso- 

 lating the three blastomeres of the three-fold type and 

 the four blastomeres of the four-fold type. Under these 

 circumstances one or two or three of the isolated blas- 

 tomeres may produce a normal embryo, but, as a rule, 

 not more than one normal embryo develops, although as 

 stated, cases of two or three embryos are also found. 

 This result can be explained on the ground that only 

 those blastomeres develop normally in which one full 

 complement or set of chromosomes is present. Boveri 

 concludes that normal development is dependent on the 

 presence of at least one set of chromosomes. Hence the 

 evidence points to the conclusion, he believes, that the 

 chromosomes are different; and that one of each kind 

 must be present to insure a normal process of develop- 

 That the results are not due to cytoplasmic differences 

 is shown by the fact that the plane of first division passes 

 through the axis of the egg, so that each blast omere gets 

 a part of the different regions of the egg. That the re- 

 sult is not due to the size of the blastomere is shown by 

 a comparison with isolated blastomeres of eggs that have 

 divided normally. Moreover, experiments with fertilized 

 egg-fragments show that normal development is not de- 

 pendent on a prescribed size relation between the nucleus 

 and the cytoplasm. 



Other objections that have been raised have also been 

 successfully met by Boveri and I can not but think, there- 

 fore, that until more valid objections can be found, 

 Boveri has made good his point. 



The experiment of fertilizing non-nucleated fragments 

 of the egg has demonstrated that a single set of chro- 

 mosomes suffices to produce normal development. Arti- 

 ficial parthenogenesis in the sea urchin has also shown 

 that the single set of chromosomes in the female pronu- 

 cleus is capable of giving rise to normal embryos. It 

 follows that as a result of normal fertilization a double 



