88 FUNDAMENTALS OF CYTOLOGY 



tion with X rays, which are capable of causing breaks in the chromo- 

 nemata. The procedure is as follows. Cells are irradiated at whatever 

 stage it is desired to ascertain the chromonema number, e.g., during the 

 telophase or the metabolic stage. They are afterwards allowed to grow 

 until the nuclei have had time to reach the anaphase of the succeeding 

 mitosis. The anaphase figures are then examined for broken chromo- 

 somes or chromatids, and from the types of abnormality observed 

 inferences are drawn regarding the number of chromonemata present 

 at the time the breakage was induced. 



The possible types of breakage and their effects upon the appearance 

 of the anaphase chromosomes are shown diagrammatically in Fig. 61. 

 If the chromosome has only one chromonema and it is broken bj^ the 

 X ray, both of the separating chromatids at anaphase should lack a 

 portion, since they were formed by splitting of a thread already broken. 

 This is a so-called chromosome break. The part lost may lie near by. 

 If there are two chromonemata at the time of breakage, a variety of 

 results may appear at anaphase: (1) a normal chromatid may be seen 

 separating from a deficient one, with a single fragment lying near by; 

 this indicates the break of but one of the two chromonemata — a chro- 

 matid break. (2) Two deficient chromatids may be seen separating, 

 with a double fragment near by ; this looks like the result of a chromosome 

 break, but it could be due to the breakage of two associated chromatids 

 by the same X-ray ''hit," It is known that two threads may thus be 

 broken even when they are more than O.Ijli apart. (3) A chromatic 

 "bridge" may appear at anaphase as a result of a reunion of the broken 

 ends of the two associated chromatids, giving a chromosome with two 

 kinetochores which may pass toward opposite poles. If there are four 

 chromonemata at the time of breakage and only one of them is broken, 

 a chromatid with two equal longitudinal halves may be seen separating 

 from one wdth unequal halves. This is a half-chromatid break. 



The results obtained with this method thus far by different workers are 

 equivocal. Some find evidence for the presence of two chromonemata at 

 somatic telophase and a division of these into four either in the late 

 metabolic stage or very early in the prophase. Others conclude that 

 there is but one chromonema in the telophase, this becoming doubled 

 late in the metabolic stage. To what extent such discrepancies are due 

 to differences in the type or condition of the materials used, or to varia- 

 tions in the procedures, remains to be determined. An interesting piece 

 of evidence is the appearance of two satellites and sometimes two nucleoli 

 side by side on a telophase chromosome. 



It has been suggested that the process of doubling may involve a 

 succession of reactions extending over a considerable period of time and 

 that various phases of the process may be affected by different agencies 



