464 THE AMERICAN NATURALIST [Vol. XLIX 



ceed normally, the chromosomes were usually distributed 

 irregularly by a three-poled or a four-poled spindle. As 

 a result three or four cells were produced at the first divi- 

 sion of the doubly fertilized egg, instead of the two cells 

 that arise after normal fertilization. Various abnormal 

 larvae were produced later. In such embryos, Boveri found 

 the organism to be divided into definite regions, thirds or 

 fourths, each part traceable to one of the three or four 

 original cells, and the cells of each part differing from 

 the cells of the other parts in their combination of chro- 

 mosomes and usually in their chromosome number. In 

 rare cases normal embryos were produced, but these were 

 more commonly developed from a doubly fertilized egg 

 which in its first division was three-celled, than from one 

 in which it was four-celled. The thought occurs at once 

 that three cells have a better chance than four cells in 

 securing a full set of chromosomes, both as to number and 

 kind. If the division were normal, each nucleus would 

 receive a full set in the case of the chromosome distribu- 

 tion to three cells, but the division is usually irregular, 

 and because of this irregularity each cell does not usually 

 secure its normal set of chromosomes. Nevertheless it 

 is clear that the embryo parts developed from the three- 

 celled cleavage stand a much greater chance of being 

 normal than those from the four-celled type, although 

 through irregularities in division an eighteen-chromo- 

 some-celled region might be formed even where the first 

 division was four-celled. 



In some cases, the embryo was completely normal as 

 regards skeleton and pigmentation in one or even two of 

 its thirds, while the remainder was entirely lacking in 

 these characters. Nearly normal embryos occurred which 

 were perfect as to parts and specific characters, but indi- 

 vidual variations which normally should have appeared 

 in separate larvae were present among the thirds. Asym- 

 metrical larvae also were formed. 



More important still are the results Boveri obtained by 

 isolating the three cells of the three-fold type and the 



