693 



r /'. ' 'A 



/'//; ; 











■ 1 ■ -..^ 



Fig. 541. — Schematic history of two homologous chromosomes during meiosis. 



From Callan, New Biology, No. 7, 1949. Penguin Books. 



a, Resting nucleus, with nuclear membrane, nucleolus, and three aggregations of nucleic acid. 



b, The chromosomes appear ; each is a single thread ; the centromeres are represented by gaps 



c, The two homologous chromosomes have started to pair. 



d, Pairing complete. 



e, Each pairing chromosome has divided into two chromatids which have formed chiasmata at three 



pomts. 

 /, Shortening and thickening (approaching end of prophase). 

 g, the nuclear membrane has broken down, the spindle has formed, and the chiasmata are arranged on 



the equator (metaphase). 

 A, Tbe centromeres are moving towards the poles, each puUing after it two chromatids (anaphase), 

 t, 1 he spmdle has elongated and the cytoplasmic cleavage furrow is approaching from the sides, 

 •fc f ^^^ ^"^^ division ; two nuclei have formed, and the cell has divided. 



k, I, m, The second division is shown for one only of the products of the first division : the result is two 



nuclei with single chromatids. 



