THE LIVING CELL 51 
like them contain 2x chromosomes, of which x chromosomes are 
contributed by the egg nucleus and x chromosomes by the sperm 
nucleus. It is apparent that if each uniting egg and sperm had 
2x chromosomes, the zygote resulting would contain twice the 
number of chromosomes as found in the body cells of the parents. 
Moreover, the body cells of the sporophyte resulting from the 
Fic. 27.—Diagram showing various stages in the reduction division of a spore- 
mother-cell of a plant. A, resting stage of the mother-cell nucleus; B, the nuclear 
skein or spirem during synapsis; C, the spirem after synapsis, showing its double 
(diploid) nature; the dotted line indicates the segmentation of the spirem into 
2 diploid chromosomes, each of which has split longitudinally in DF, the diploid 
chromosomes on the equator of the spindle of the first (heterotypic) division; F, 
late anaphase; G, metaphase of the second or homotypic division; i; late anaphase 
of same, two haploid chromosomes approaching the poles of each spindle; I, the 
four daughter cells (spores) of the tetrad. (After Gager.) 
division and redivision of the odspore would contain 4x chromo- 
somes. This does not occur, for at a certain stage in the life 
history of every sexually reproducing plant between each zygote 
formation and the next, a reduction division takes place whereby 
the 2x chromosomes are reduced to x chromosomes. ‘This 
reduction in the chromosome number takes place in the two 
nuclear divisions of spore -mother cells to form pollen grains and 
embryo sacs with the result that each of the starting points of the 
gametophyte generation have their chromosomes reduced to the 
x or haploid number. 
