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PHYSICAL BASIS OF HEEEDITY 



ber that developed into normal plutei with the number 

 of plutei from one-fourth normally fertilized blastomeres. 

 From the latter a large proportion give rise to normal 

 embryos, from tlie former normal embryos are rarer. 

 Their greater rarity, Boveri thought safe to attribute to 

 the chromosomal deficiencies present in most of such iso- 



FiQ. 98. — Scheme showing dispermic fertilization of the egg of the sea urchin with the 

 subsequent irregular distribution of the chromosomes. (After Boveri.) 



lated blastomeres. He suggested that the chance of a 

 blastomere developing normally depends on its having 

 at least one full set of chromosomes. For these triploid 

 sea urchin eggs with three times 18 chromosomes, the 

 chance of one full set of chromosomes getting into each 

 blastomere is, according to Boveri ^s calculation, only one 

 to 10,000. The chance of getting at least one chromosome 

 of each kind in one cell is greater. He concluded that the 

 few embryos he obtained came from quadrants that had at 

 least one haploid set of chromosomes. There is, however, 



