40 MITOSIS : THE CONSTANCY OF THE CHROMOSOMES 



origin of nucleoli from the particular unspiralised segments which 

 we call secondary constrictions. At prophase such correspondences 

 are difficult to trace, because the nucleoli often fuse at this stage, 

 especially in meiosis, but the " nucleolar chromosomes " can still 

 be recognised by their association with them. The nucleolus may 

 lie round the chromosome, separating it into pieces (Kaufmann, 

 1933 on Drosophila nielanogaster) or it may be merely attached to 

 it by a thread (Dobzhansky, 1934, on D. pseiido-ohsciira). This 

 difference is found between mid and late prophase in the meiosis 

 of plants. As to whether there are non-spiralising segments giving 



V. narbonensis (l4) ^- ^^^''"^* ^^^J 



V. Faba (12j ^- ^^"squinez (14) 



Fig. 8. — Anaphase chromosomes and telophase nuclei in four 

 species of Vicia, showing the position relationship of secondary 

 constrictions and nucleoli (after Heitz, 1931 h). 



constrictions at metaphase apart from the segments adjoining 

 nucleolar organisers, we have as yet no evidence. But it is certain 

 on the other hand, that small nucleoli may leave no trace of con- 

 striction at metaphase. 



(iii) In the Resting Stage : the Theory of Permanence. The 

 numbers, sizes, shapes and constrictions of the chromosomes 

 appearing at mitosis are constant. In the nuclei of an individual 

 or of a group of individuals descended by mitosis from a single 

 nucleus, apart from such special accidents as will be considered 

 later, the same " complement " is found. With sexual reproduction 

 other changes may occur, but these are not common enough to 



