THE POLYPLOID SERIES IN OSMUNDA 



together laterally of every chromosome with the exactly equivalem homologous partner. 

 Whilst this is occurring the chromosomes are very long and thin, as may be seen by a 

 glance at Figs. 19 and 20, but when pairing is complete they appear to shorten and 

 thicken in a most remarkable way (cf. Fig. 22). This change of shape is partly due to 

 genuine shrinkage but is chiefly caused by the fact that each chromosome becomes 

 coiled into a spiral configuration (Fig. 21) which is commonly referred to as spiral 



Q« ■■HP 



Fig. 20. Paired chromosomes at the stage known as pachytene (literally the fat thread stage), a, a com- 

 plete bivalent in the diploid; b, a trivalent in the triploid; c, a quadrivalent in the tetraploid. 

 Permanent acetocarmine preparations, x 2000. a after Manton (1939); b after Manton (1945). 



Structure but which can only be clearly seen after special treatment and which was 

 therefore for long disregarded. It will not often be necessary to refer to spiral structure 

 in the course of this book, though it is perhaps worth mention at this point, since 

 otherwise the changes of shape which affect a chromosome during prophase appear 

 unnecessarily mysterious. 



the chromosomes are assembled on the spindle, anaphase is the stage at which movement to the poles takes 

 place, and telophase denotes the stage at which resting daughter nuclei are being reformed. It is not 

 customary to subdivide these stages further for mitotic divisions, but the prophases of meiosis are so 

 complex that a whole sequence of substages have been recognized and given separate names. The most 

 important of these are leptotene, zygotene, pachytene, diplotene and diakinesis, after which metaphase of the 

 first meiotic division sets in. Detailed description of these stages will be found in many elementary text- 

 books of cytology or genetics, but for the purpose of this book the descriptive notes given in this chapter 

 for leptotene, pachytene and diakinesis should suffice. The only other cytological nomenclature which might 

 perhaps cause confusion to a non-cytological reader concerns the appellations of the two meiotic divisions. 

 In the older literature the names heterotype and homotype were applied to the two nuclear divisions which 

 constitute meiosis ; these words are, however, becoming superseded by the rather simpler designation of 

 'first meiotic division' and 'second meiotic division', and the latter practice will be adhered to through- 

 out this book. The numerical chromosome reduction by means of chromosome pairing occurs at the first 

 division. The second division involves the longitudinal separation of half-chromosomes as in a somatic 

 mitosis. The result of the two divisions is a tetrad of four nuclei each with the reduced chromosome 

 number. 



34 



