THE FUNDAMENTALS OF MENDELISM 37 



interchanges of partner among the threads. Each group of two chromo- 

 somes (four chromatids) is known as a bivalent and the interchanges of 

 partner are chiasmata. 



{e) Diakinesis. — ^The final stage before the chromosomes arrange 

 themselves on the metaphase plate. The chromosomes have been con- 

 tracting during the whole prophase and are now quite short and thick; 

 as the prophase is more complicated and lasts longer than in mitosis, 

 the contraction has been going on longer, and the chromosomes are 

 shorter, perhaps only a third as long as in mitosis. 



At metaphase each bivalent has two centromeres, one from each 

 chromosome, and these become arranged so as to lie symmetrically on 

 either side of the equatorial plane of the spindle. Their mutual repul- 

 sion gradually forces them apart (anaphase), unravelling the chiasmata 

 also, each centromere carrying with it towards the poles two chroma- 

 tids which make up one whole chromosome. After a telophase, resting 

 stage nuclei are formed. 



The first division is normally followed by a second division in which 

 the chromosomes appear again still showing the split which they 

 developed in pachytene, and still with undivided centromeres. A normal 

 mitosis follows, the anaphase separation beginning when the centro- 

 meres eventually divide. This is sometimes spoken of as the equational 

 division as opposed to the reduction division, but we shall see (p. 103) 

 that this is misleading; another pair of names is heterotype division 

 = first division, homotype division = second division. 



The daughter cells of the second division are transformed directly 

 into gametes in most animals. The pairing of two whole chromo- 

 somes together in zygotene, followed by their separation to different 

 poles in anaphase, has brought it about that the daughter cells each 

 contain only one of each kind of chromosome and has accomplished the 

 reduction in number which we spoke of above. The relation between 

 the mechanisms of meiosis and mitosis is discussed on page 113. 



Fig. 3. Meiosis in the grasshopper Melanoplus femur-rubrum. — A-E, Optical 

 sections of prophase stages; A Leptotene, with unsplit chromosomes. 6, Zygotene, 

 pairing in progress, C Pachytene, pairing complete, D and £ stages of diplotene, 

 the chromosomes splitting and falling apart into loops. Fthe chromosome comple- 

 ment drawn separately at early diplotene; note the exchanges of partner or 

 chiasmata by which the threads are held together. The X chromosome on the 

 right shows precocious condensation. G the chromosomes further contracted 

 at diakinesis. H metaphase chromosomes seen in side view. / Early anaphase in 

 side view. J Telophase of first meiotic division. K Metaphases (polar view) of 

 second meiotic division. All magnified about 2,000 times. 



(From Hearne and Huskins.) 



