256 introduction' TO CYTOLOGY 



pone consideration of certain problematic matters until later in the 

 chapter.^ 



As the prophase of the first meiotic mitosis begins, the reticulum of 

 the meiocyte nucleus transforms into the diploid number of long and very- 

 slender threads (leptonema stage) (Fig. 150, 1). These thin threads 

 represent essentially the chromonemata of the chromosomes. Owing to 

 their attenuation and the scarcity of enveloping substances they may show 

 clearly their small chromomeres and other structural details. Their 

 doubleness is a matter of debate. Although considerable evidence has 

 been brought forward to show that in some cases at least they are already 

 split at this time, or even in the telophase of the last premeiotic mitosis,^ 

 the microscope in other cases reveals no trustworthy evidence of double- 

 ness. The threads may lie without any regular orientation in the nucleus, 

 or, especially in animals, their ends may be directed toward one side, 

 forming what has been called a "bouquet." 



The leptonema threads now begin to conjugate in pairs (synapsis). 

 This involves a series of movements which are not well understood but 

 which in some way bring each chromosome where it can enter into close 

 union with its homologue in one or more regions {zygonema stage). 

 Synapsis seems to begin at certain points in the threads, notably at their 

 ends, and then gradually to extend throughout their length. While this 

 is in progress the nucleus may show both paired and unpaired threads; 

 this amphitene condition is well shown in bouquets. During these stages 

 the threads show a notable tendency to shrink and collapse into a more 

 or less tight knot under the influence of fixatives. This shrinking is 

 known as synizesis. In most cases it is obviously an artifact due largely 

 to inadequate fixation of the karyolymph, but in some a certain amount 



^ No attempt can be made in a work of this scope to give a complete summary 

 and classification of all the interpretations that have been put upon the meiotic 

 phenomena. Only enough will be presented to afford a starting point for a study of 

 this complex subject. For a review and criticism of all views expressed up to 1910, 

 see Grcgoire (1905, 1910). The subject may be followed further in the more recent 

 general accounts of Agar (1920a), Doncaster (1920a), Tischler (1921-1922), McClung 

 (1924), Wilson (1925), Belaf (1928), Reuter (1930), and Darlington (1931f/, 1932a). 

 Useful lists of works on somatic and meiotic mitosis in angiosperms are given by 

 Picard (1913) and Ruys (1925); see also Schiirhoff (1926). 



Meiosis as outlined in the following paragraphs was first described by von 

 Winiwarter (1900), A. and K. E. Schreiner (1904 et seq.), and Marechal (1904 ei seq.) 

 for animals; and for plants by Dixon (1895), Gregoire (1904, 1907), Berghs (1904, 

 1905), Rosenberg (1905, 19076, 1909a), C. E. Allen (19056c), and J. B. Overton 

 (1905, 1909). Many of the stages have been observed in living cells: see Chambers 

 (1924, 1925) on Dissosteira, Chodat (1924) on Gymnadenia, Lenoir (1927) on Lilium, 

 and Belaf (1928) on Chorthippus. Recent detailed accounts of meiosis are cited in 

 subsequent footnotes. 



»W. R. B. Robertson (1919, 1920, 1921, 1931a) and McClung (1924, 1927a, 

 1928a) on Orthoptera, Kaufmann (1926a, 1931a) and Nebel (1932) on Tradescantia 

 and Rhoeo. The significance of these cases will be discussed later in the chapter. 



