NUCLEAR CYCLE 



23 



optical section through the coiled threads which are later clearly 

 distinguishable (Belar, 1929 h). 



In the most favourable material it is possible to see at the earliest 

 stage that the threads are double, and it is probable therefore that 



M. £.A. Lj\. T 



Fig. 4. — The mitotic cycle in a nucleus with four recognisable 

 chromosomes, one having a subterminal centromere has a 

 nucleolar constriction. U.S., resting stage. E.P. and L.P., 

 early and late prophase (with dividing centrosome and disappear- 

 ing nucleolus). M., metaphase (with spindle). E.A. and L.A., 

 early and late anaphase. T., telophase. 



this is their universal characteristic throughout prophase (Ch. IX). 

 The double threads can be traced in their development until 

 metaphase, when they are recognised as chromosomes. The two 

 single threads of which they are composed may be described as 

 chromatids. This term was originally used by McClung to describe 

 the single threads at meiosis, but since the single threads (being 



