18 HEREDITARY CHARACTERS 



The whole of this process of division is very rapid, and 

 from the first signs of preparation for division until the two 

 daughter cells are completely separated from each other, 

 probably but a few minutes elapse in the case of some 

 organisms. The process is particularly rapid in the higher 

 animals. Divisions succeed each other with the greatest 

 possible rapidity in the earlier stages in the development of 

 the embryo, that is, from the time the ovum divides into 

 two cells until the formation of the organism has reached a 

 comparatively advanced stage. 



A consideration of the mode of division seems to show 

 that the only parts of the cell that divide in a selective 

 manner are the chromosomes ; all the rest of the cell, 

 excepting the centrosomes, seems to divide simply according 

 to bulk, the ground substance of the nucleus and cytoplasm 

 being mingled at the time division takes place. This fact 

 has led very many observers to believe that in the chromo- 

 somes and chromatin are contained all the hereditary char- 

 acters of the race. Chromatin has in fact been identified as 

 Nageli's idioplasm. The fact that the chromatin in its 

 envelope of linin generally seems to be in the form of 

 minute granules, has led to the further conception that 

 each of these granules represents a character, more particu- 

 larly as when these granules are observable in the chromo- 

 some, each granule is sometimes seen to split individually, 

 half of each granule being contained in each daughter 

 chromosome. 



This theory is at first sight an extremely fascinating one. 

 An exact representative half of every chromosome in the 

 fertilised ovum is handed on to each daughter cell. The 

 same happens again in the daughter cells, and subsequently 

 in every division that takes place in the building up of the 

 body of the organism. Thus in the adult organism every 

 cell contains a similar number of chromosomes to that 

 contained in the fertilised ovum, and moreover each of 

 these chromosomes is derived from a similar individual 

 chromosome that was present in the ovum. The mode of 



