90 MEWS IS IN DIPLOIDS AND POLYPLOIDS 



Both the granules and the thread are single so far as observation can 

 show. In favourable material of some animals the threads can be 

 counted from their centromeres and shown to be present in the 

 diploid number, as in somatic mitoses (Gelei, 1921 ; Janssens, 

 1924). This stage with separate single undivided threads is called 

 the leptotene stage. 



The chromosome threads may remain freely distributed in the 

 cell, as in most of the higher plants, or they may gradually orientate 

 themselves so that one or both ends of each thread lie towards one 

 side of the nucleus (near the centrosome where one is present). 

 They are then said to be polarised, and as long as they remain so. 



A B 



Fig. 21B. — Diagram to show the difference between the method of 

 pairing at zygotene with and without polarisation. A, as in 

 Stenobothrus ; B, as in Tulipa. 



that is, until pachytene, they present the appearance to which the 

 name " bouquet " stage is given (Digby on Osmunda, 1919 ; Gelei 

 on Dendroccelum, 1921 ; Janssens on Mecostethus and Stenobothrus, 

 1924; Bela.T on Actinophrys , ig26 ; KihsiTa.onRumex,ig2y a; Belar 

 on Tradescantia, 1929 h). 



The single threads soon begin to associate in pairs side by side. 

 If they are freely distributed in the nucleus, their association some- 

 times takes place at random, i.e., in different parts at the same time. 



The bouquet stage has been regarded as a possible characteristic 

 of chromosome pairing (Gelei, 1921), especially on account of its 

 absence in certain hybrids without pairing (e.g., Pygcera, Federley 

 1912), and in certain parthenogenetic organisms without pairing 



