THE FUNDAMENTALS OF MENDELISM 



41 



chromatin, but by possessing the same number of chromosomes with 

 the same sizes and shapes. This is most strikingly shown in organisms 

 in which the chromosome complement has become altered from that 



Fig. 6. Nucleoli and Chromocentres 



in Vicia Faba. — A One daughter nucleus in 

 a late anaphase stage; the nucleolus-bearlng 

 chromosomes are drawn black, and the 

 nucleolus-organizer is visible as a non- 

 staining gap in the chromosome. B The 

 resting stage nucleus with two nucleoli 

 and (smaller) chromocentres. 



(After Heitz.) 



normal to the species, when the alterations are found to be common to 

 all the cells of the organism and can indeed be inherited. 

 As well as this general evidence of repetition of shape, there is 



Fig. 7. Constancy of Chromosome Shape.— (1) Mitotic metaphase of Crepis 

 capillaris; (2) of C. tectorum; and (3) of the hybrid between them, the individual 

 chromosomes being recognisable, Ta, Tb, Tc, Td from tectorum and Ca, Cc, Cd 

 from capillaris; the only change which has occurred is that Td has lost its trabant 

 (terminal knob). 



(From Darlington, after Hollingshead.) 



evidence that each chromosome arises in a prophase in the same posi- 

 tion and with the same pecuHarities as it had when it disappeared in the 

 preceding telophase. Thus shortly before the chromosomes disappear 



