Cell Division and Heredity 61 



they will push inwardly the nuclear membrane 

 whenever they encounter the latter. 



The specific mode of division of the chromo- 

 somes will remain an interesting subject for fur- 

 ther study. It was formerly thought that division 

 of the chromosomes was brought about by the cen- 

 tral bodies, but numerous cases have been ob- 

 served where complete division of the chromo- 

 somes take place without any division of either 

 the central bodies, the cytoplasm, or the nuclear 

 membrane, so that it is now generally admitted 

 that division of the chromosomes takes place 

 spontaneously as a result of internal forces alone. 

 In fact, there are numerous cases where the spi- 

 reme thread, which subsequently forms the chro- 

 mosomes, will separate completely into two lateral 

 halves before any of the astral rays have pene- 

 trated into the nuclear region, which proves con- 

 clusively that the astral rays or central bodies 

 could not have caused division of the chromo- 

 somes. The fact that the daughter chromosomes 

 move away from each other most rapidly at points 

 where they have become attached to the spindle 

 fibers may be due to some sort of surface phe- 

 nomenon, but does not prove that the original 

 cleavage of the chromosomal thread was caused 

 by these spindle fibers. 



