210 HEREDITARY TRANSMISSION [CH. 



size of the nuclei on the two sides is quite inexplicable. 

 Hybrids also practically never show the maternal (Sphaer- 

 echinus) characters developed to the extent to which HE REST 

 found them in the maternal halves of his larvae. 



Experiments such as those described, which by no means 

 exhaust all the evidence available, give strong indications 

 that the factors for hereditary characters are borne by the 

 nucleus rather than by the cytoplasm. This conclusion is 

 strengthened by observations on the chromosomes in par- 

 ticular, as distinct from the nucleus as a whole. 



An observation of the same general type as those just 

 described is that of BALTZERonthe behaviour of the chromo- 

 somes in reciprocal crosses between the Sea-urchins Sphaer- 

 ecbinusandStrongylocentrotus. He found that eggs of Sphaer- 

 chinus fertilised by Strongylocentrotus sperm develop regu- 

 larly, with no abnormality in the behaviour of the chromo- 

 somes, and the plutei from this cross are intermediate 

 between those of the paternal species in their skeletal 

 characters. In the development of embryos from the con- 

 verse cross, however (Strongylocentrotus eggs by Sphaerechi- 

 nus sperm), a remarkable irregularity takes place in the first 

 and second segmentation divisions of the cross-fertilised egg. 

 In these divisions altogether fifteen or sixteen chromosomes 

 are omitted from the division spindles. They split in the 

 equatorial plate of the first division, but fail to divide com- 

 pletely, and some are left behind on the spindle in the 

 anaphase stage, while others are carried with the normal 

 chromosomes to one or other daughter nucleus. In the 

 second division, those which have been included in the 

 nuclei reappear as split rods, and are left out when the four 

 daughter nuclei are formed. The haploid numbers of the 

 parent species are twenty for Sphaerechinus and eighteen for 

 Strongylocentrotus, and after the elimination of the abnormal 



