THE MECHANICS OF THE CHROMOSOMES 



117 



is different in different organisms but that splitting takes place some 

 time after the telophase and before the prophase, e.g. in locusts about 

 24 hours before. This seems a complete vindication of Darlington's 

 original form of the hypothesis.^ 



The critical evidence in this line of argument is provided by chromo- 



y 



RESTING 



m 



S TAGE 



IPROPH.J 



> 



Fig. 55. The Time of Division of the Chromosomes. — Chromosomes are 

 broken by X-rays at various intervals before entering mitosis and the metaphase 

 figures examined. If the break occurs before the chromosome splits into two 

 chromatids, one may fmd figures v/here two chromosomes have been broken and 

 rejoined with each other (lower row). But if the break occurs after splitting into 

 chromatids, pseudo-chiasmata can occur between the four chromatids from 

 non-homologous chromosomes which happen to be lying near one another (upper 

 row). Similarly, early irradiation can break a chromosome in two, later irradiation 

 will only affect one of the two chromatids (top right of each drawing). 



(After Mather.) 



On the right is a drawing of a chromosome at mitotic metaphase, showing its 

 alleged quadripartite structure, in the injured region three of the four half- 

 chromatids of which the chromosome is supposed to consist have been destroyed 

 by X-rays. 



(After Nebel.) 



some figures which are large enough to be incontrovertibly established. 

 It has been stated, however, that changes on a smaller scale occur 

 which lead to the hypothesis that the fundamental structure of the 

 chromosome shortly before mitotic prophase is not that of a pair of 

 chromatids but of four half-chromatids, and figures have been pub- 

 lished showing three of these wiped out by X-rays and only one left.^ 

 However, one is here again attempting to discriminate between two 



^ But cf. Huskins 1937, where he gives up his own theory but does not accept 

 Darlington's. * Nebel 1936. 



