REPRODUCTION AND GROWTH 173 



body the number of chromosomes is always the same and it is 

 also the same for all individuals of the same morphological 

 category. The number varies from 2 to about 150, but the most 

 frequent values are about 40 to 60. The chromosomes are very 

 small and can only be seen well under the highest magnifications 

 of the microscope. 



The appearances seen under the microscope when the cell 

 divides are (typically) as follows (there are very many divergent 

 details) : 



(i) The typical number of chromosomes form ; (2) then each 

 of them becomes double (by reason of its splitting lengthwise) ; 

 (3) the two series of half-chromosomes become arranged as a 

 " plate " in the equatorial region of the cell ; (4) they are then 

 drawn apart so that they come to lie towards the polar regions ; 



(5) the nuclear membrane re-forms round each group of chromo- 

 somes so that the cell comes to contain two nuclei : at the same 

 time it begins to suffer constriction round its equatorial region ; 



(6) finally the constriction becomes deep and narrow and the cell 

 divides into two daughter-cells each containing a nucleus. 



The cells then differentiate to some extent and each of them 

 usually grows to the size of the parent-cell. 



Evidently the essential nature of this process is the exact 

 division into two of all the significant parts of the cell. The 

 chromosome is really a row (" linear series ") of smaller units 



(the chromomeres), thus — ,OOOOCOOCOC — and each of the 



units must be (halved for, by hypothesis, they are all different). 

 So the chromosome must split lengthwise thus : 



— plane of the " splitting." 



It is believed that other essential bodies in the cytoplasm also 

 divide, like the chromosomes do. Obviously the process mitosis 

 ensures that every one of the essential things in a cell doubles 

 itself so that when the parent-cell divides a complete '' outfit " 

 of parts goes into each daughter-cell. These outfits are then 

 reassembled as complete nuclei and cytoplasmic apparatus and 

 mass-growth occurs, when the daughter-cells assume all the 

 parental form and characters. There must be a mechanism of 

 mitosis, but we have no knowledge as to its nature. 



