MEIOSIS 



11 



ing portions are exchanged being a consequence of their orderly arrange- 

 ment in synapsis. On the other hand, theories have been propounded to 

 account for crossing over without actual breakage. The idea is that new- 

 threads are developed parallel to the old ones immediately after the 

 chromomeres of each synaptic mate have doubled by division in the 

 pachytene stage, and that when the synaptic mates have a sharp twist at 

 any given point two of the resulting chromatids may contain chromomeres 



;X: 



II (I 

 II II 

 II ■ p 



p I 



p p 



p I p 



12 3 4 5 



Fig. 87. — Diagram illustrating theory of crossing over involving formation of new 

 strands between newly formed chromomeres. 1, original chromosomes. 2, chromosomes 

 with half-twist. 3, chromomeres doubled. 4, new strands developing; between the third 

 and fourth pairs of chromomeres they result in crossover chromatids. 5, the tetrad of 

 chromatids after crossing over. {Based on theory of J. Belling.) 



from different mates on either side of this point because the new chro- 

 momeres form the new unions with their nearest neighbors (Fig. 87). 

 Under this theory, crossing over is a concomitant of chromosome division. 

 Finally, there is the problem of the relation of meiosis to the phenomena 

 of genetics. So far as major features such as synapsis, disjunction, 

 random assortment, and crossing over are concerned, this problem has 

 been solved, as will be shown in the later chapters of the book. Numer- 

 ous other questions, however, remain open, and it is hoped that these 

 will be answered by fui-ther refinements in our knowledge of the constitu- 

 tion and behavior of meiotic chromosomes. 



