HKRKDITV AND VARIATION 





(z) Each chromosome separates into exactly equal halves (chromatids) 

 which arc distributed to opposite poles <>t the dividing cell. 



Hence, (3) each daughter nucleus receives eld exactly equal share "f tin- 

 chromosome material of the parent QU< leus (Fig. 420). 



Proph 



ase 



B 



Y 



AtetepAase 



1 



(division cft/ip 



nuc/eus \ 



Anaph 



ase 



V 



Te/op/iase 



(ftro nuc/ei) 



a, atfacnmenf c<7/7sfr/cf/0/z . 



Fie. 429. 

 Diagram showing the changes undergone by two chron. 111 the 



course of mitosis. (From Crane and Lawrence alter Darlington.) 



If, as is generally assumed, the heritable qualities of the organism are local- 

 ised in the chromosomes— as so-called genes— mitosis dearly provides a 

 mechanism by means of which these qualities are transmitted unchanged 

 each somatic division. The fact that new individuals produced by veg« tative 

 propagation (Chapter XIII) exactly resemble the parent plant bears out this 



conclusion. 



Meiosis. 



The formation of tetraspores (such as pollen-grains, spon a t Pteridoph 

 and Bryophytes, tetraspores of Rhodophyceae, ascospores and basidiospon 

 involves two divisions of the nucleus accompanied by one division of the 

 chromosomes; as a result, the chromosome number is halved. Mine four 

 tetraspores are produced from each spore mother cell, the whole proa 

 is often described as a tetrad-division. 



