40 HEREDITARY CHARACTERS 



adult individual is present in the single cell produced by 

 this fusion (the fertilised ovum). There is no conclusive 

 evidence as to the nature or situation in the cell of 

 hereditary substances representing racial characters. 



4. Certain parts of the cell divide in a selective manner 

 which is not a mere division in bulk. These are : 



(a) The chromosomes appear to be handed on individu- 

 ally from the fertilised ovum to all the cells building 

 up the body of the organism. They are, however, 

 distributed in an alternative manner to the cells 

 that are destined to be thrown off as sperms or 

 ova. Also it has been shown that embryos pro- 

 duced from one nucleus only and possessing only 

 half the normal number of chromosomes exhibit 

 all the racial characters. Apparently, therefore, 

 if individual chromosomes represent . hereditary 

 characters, such characters cannot be common to 

 all the individuals of the race, but must be char- 

 acters that are transmitted in an alternative manner 

 from parent to offspring. 



(6) The centrosomes, when present, are handed on indi- 

 vidually from one cell generation to another, accord- 

 ing to some observers. Others, however, dispute 

 this, and say that in many organisms they are 

 formed de novo prior to each cell division. In any 

 case, centrosomes are absent in the higher plants. 

 (c) Certain cytoplasmic structures may divide individu- 

 ally and be handed on from one cell generation to 

 another. This is common in the case of certain 

 cytoplasmic structures in many plants and in a 

 few animals. Wider claims have recently been 

 made in this direction for cytoplasmic structures 

 by at least one very eminent biologist. 



Note on Centrosomes. Besides being absent in the higher 

 plants, centrosomes are said by some observers to pass out 

 of existence at the end of one cell division, to be reformed 



