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INTRODUCTION TO CYTOLOGY 



altered (Fig. 177). Finally, one or more fragments additional to a normal 

 complement may be present after a number of generations (Fig. 178). 

 Our knowledge of the actual mechanism of fragmentation is rather 

 meager. Available evidence, including the fact that portions of chromo- 

 somes are much more often translocated or inverted than left as free frag- 

 ments, suggests that contacts or entanglements between chromosomal 

 threads are in part responsible (Fig. 179). It is not uncommon to observe 

 prophasic threads interlocked and stretched as if a rupture or exchange 

 were soon to follow at the point of contact. The frequency of transloca- 

 tion and related alterations can be notably increased by the use of X-rays, 

 even when the nuclei treated are known to be in the metabolic condition. 

 For example, kernels of barley and maize, after being irradiated when in 

 the dormant state, give rise to individuals showing a relatively high 



Fig. 180. — Breaking of chromosome after failure of distal ends to disjoin in living spermato- 

 cyte of Chorthippus. The changes shown occupied 40 minutes. {After B'elaf, 1929a.) 



percentage of translocations in the multiplying cells (Stadler, Randolph). 

 Similar results follow the irradiation of mature pollen. In Drosophila 

 such aberrations can be induced by irradiating the mature spermatozoa. ^ 

 The natural inference from such data is that the chromosomal threads 

 persist in some form throughout the entire nuclear cycle, and that X-radi- 

 ation increases the frequency of fragmentation and the like by inducing 

 localized alterations in the position, consistency, and tension of these 

 threads. In Zea certain translocations have been shown to be due to the 

 occasional association of non-homologous elements in the meiotic pro- 

 phase (p. 276). 



Another mode of fragmentation is that in which a small portion of one 

 chromosome is torn away by its synaptic mate in the first meiotic ana- 

 phase. This has been observed in a living spermatocyte (Fig. 180), 

 and a similar process is strongly suggested by a Zea sporocyte observed by 

 Randolph ; but it is probable that mitotic derangements of this sort play 

 only a minor role in the production of alterations in the chromosome 

 complement. 



2 MuUer (1927), Muller and Altenburg (1930). 



