58 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



chromosomes at a later stage was correspondingly increased. Since it 

 occurs in some species and not in others, this developmental property 

 is evidently genotypically controlled, and the character may be 

 described as the property of aggregation of the smaller chromosomes 

 into larger ones, which is irreversible in the germ tract and reversible 

 in the somatic nuclei, for the related species have the unaggregated 

 condition in all their nuclei (Boveri, 1904). 



(b) This requires that the centromeres of all the chromosomes 

 in an aggregate, save one, shall be functionally suppressed, the 

 breaking-up of the aggregate then being due to the revival of their 

 activity. How this probably happens we shall see later. 



In Ascaris, at the same time as fragmentation, and in Miastor 

 and Sciara (Diptera), in the same circumstances, portions of the 

 chromosome complement are left on the equatorial plate at anaphase. 

 The portions lost are the distal ends of the fragmenting chromo- 

 somes in Ascaris and half the number of the chromosomes in Miastor. 

 This change is necessarily irreversible [Miastor, Kahle, 1908 ; 

 Hegner, 1914 ; Ascaris lumbricoides, Boveri, 1904 ; Sciara, Du 

 Bois, 1933). 



(c) In Ascalaphus libelluloides (Naville and de Beaumont, 1933) 

 there occurs a fragmentation of two homologous chromosomes, 

 the sex chromosomes. This change occurs gradually in mitosis 

 and it is variable. It is also reversed entirely at the second meiotic 

 division. In the hemipteran Acholla (Payne, 1910) the X chromo- 

 some seems to have broken up into five elements, but its breakage 

 is permanent. Aggregation in Phragmatobia and Philosamia is 

 probably also genotypically controlled, but diminution in Drosophila 

 affecting particular chromosomes is more probably structurally 

 controlled (cf. D., 1932). 



(iii) Conclusion. These observations confirm the a priori views 

 set out earlier. The relative form and behaviour of the chromosomes 

 at mitosis in the same complement is determined by the length and 

 linear structure of the chromosome thread which exists permanently 

 at division and in the resting nucleus. Under different physiological 

 conditions, however, the same thread will behave differently in 

 regard to (i) the bulk it assumes at metaphase ; (ii) the degree of 

 spirahsation it undergoes during prophase ; (iii) the aggregation 



