CHAPTER II 



MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



The Resting Nucleus — The Behaviour of the Chromosomes — Abnormal 

 Forms of Mitosis — The Structure of the Chromosomes — Constrictions — Spiral 

 Structure — Permanence and Division of the Chromosomes. 



Make me to see't ; or at the least so prove it 

 That the probation bear no hinge or loop 

 To hang a doubt on. 



Shakespeare. Othello, Act III, Scene 3. 



I. THE RESTING NUCLEUS : NUCLEOLI 

 The nucleus is a body denser than the surrounding cytoplasm, 

 for in centrifuged cells it passes to the periphery of the cell 

 (Beams and King, 1935). It is also more viscous, and dehydration 

 shrinks it less than the cytoplasm. It is usually globular, except 

 in certain specialised tissues or when it degenerates. It is a smooth- 

 surfaced body with a sharp boundary between it and the cytoplasm. 

 There is no reason to suppose that it is enclosed by a membrane 

 distinct from the materials within and without it, although such a 

 membrane may be formed under the stimulus of micro-dissection. 

 On the other hand the cychcal changes that will be described in the 

 life of the nucleus are not related to similar changes in the cytoplasm. 

 Their interface must, therefore, behave as a kind of semi-permeable 

 membrane separating two independent systems {cf. Ch. XII). 



The living nucleus is often optically homogeneous, but, as will 

 be seen, this does not necessarily imply any physico-chemical 

 homogeneity. 



The size of the nucleus is subject to great variation, not only as 

 between different organisms, but also within the same organism. 

 In bulk it may best be compared with the chromosomes into which 

 it resolves itself at mitosis, for these are of relatively constant size. 

 The micronucleus of the infusorian is little if at all bigger than the 

 mitotic chromosomes. The nucleus in the male gametes and in 



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