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Fig. 209. A diagrammatic representation of chromosome behavior during reduc- 

 tion division and the formation of microspores and megaspores. 



I, reduction division ending in cells represented by J or K in which the nuclei 

 contain but one-half as many chromosomes as cell C. The early stages in reduction 

 division are essentially like those of vegetative cell division illustrated in A and B 

 in Fig. 207. Hence we may begin with C above, a spore mother cell in which the 

 chromosomes have become distinct; D-H, the chromosomes assemble at the 

 equator of the spindle in pairs, each paternal chromosome pairing with its 

 homologous maternal chiomosome; I, the mates of each pair of homologous 

 chromosomes separate and begin to migrate to opposite poles of the spindle; J, 

 chromosomes at the poles of the cell. Each daughter nucleus contains but one-half 

 as many chromosomes as the parent nucleus in cell C. 



II, K-N, the two cells formed by reduction division usually divide immediately 

 by ordinary cell division resulting in a tetrad of spores ( microspores or megaspores ) . 

 The longitudinal splitting of the chromosomes seen in the division of each of these 

 two cells usually begins before reduction division is completed as shown by the 

 doubleness of the chromosomes in E-K. Drawn by Lois Lampe. 



these chromosomes. When it unites with a sperm the resulting triple- 

 fusion nucleus has three sets of chromosomes. 



If we represent the number of chromosomes in the megaspore and in 

 each of the eight nuclei of the embryo sac bv n, then the number of 

 chromosomes in the fusion nucleus is 2n; but in the triple-fusion nucleus 

 and in every cell of the endospenn of the seed it is 3n. Since the fer- 

 tilized egg has a set of chromosomes from the egg and another from the 



