CHAPTER 5 



The Mechanics of the Chromosomes^ 



I . The Relation of Meiosis and Mitosis 



We must now expand the summary description of mitosis and 

 meiosis given in Chap. i. 



The most important difference between the two processes is that in 

 mitosis the prophase chromosomes appear in the diploid number of 

 paired threads, while in meiosis they appear in the diploid number of 

 single threads, which then come together in pairs to form the haploid 

 number of double bodies. This pairing side by side of the meiotic 

 chromosomes is known as parasynapsis and is generally accepted at 

 the present day. Until a few years ago it was disputed by some cyto- 

 logists, who maintained that the early prophase chromosomes, both in 

 mitosis and meiosis, were joined by their ends into a continuous thread 

 or spireme, which was double in mitosis, but single in meiosis. This 

 thread was supposed to segment into the diploid number of paired 

 chromosomes in the mitotic prophase, but in meiosis into the haploid 

 nimiber of bodies each of which consisted of two homologous chromo- 

 somes paired end to end (telosynapsis); these two chromosomes then 

 became bent at their point of junction so that they lay side by side in 

 zygotene. In another variation of the theory it was supposed that the 

 thread was at first single both in mitosis and meiosis, and that in 

 mitosis also the side-by-side arrangement of the daughter threads, 

 which can be seen in later stages, was arrived at by the doubling back 

 on themselves of two chromatids which originally were paired end to 

 end. The hypothesis of telosynapsis, then, supposed that both in 

 meiosis and mitosis the chromosomes first appeared as a continuous 

 spireme which became segmented between each chromosome in 

 mitosis and between alternate chromosomes in meiosis. No explanation 

 was offered for this difference in the method of segmentation, but it was 

 the lack of a satisfactory observational basis which led to the abandon- 

 ment of the hypothesis. The existence of a continuous spireme could 

 only be plausibly suggested in those organisms where the large number 

 of chromosomes made observations of early prophase very difficult; in 



^ General references: Belar 1928, Darlington 1937, Geitler 1934, Sharp 1934, 

 White 1937, Wilson 1928. 



