TIIR SPERM A TOPJI VTA 



359 



tive corolla, composed of petals; its pollen-producing stamens, and 

 its ovule-bearing pistil. The pistil may be a single carpel (which 

 has grown about the ovules and enclosed them); a number of 



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• • 





I m f' 



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Fig. 230. — Development of the embryo sac, and fertilization, in the squash. 



A, megaspore mother-cell, which is situated in the middle of a very young ovule. 



B, the four spores which develop from the mother-cell. Three of these degener- 

 ate and disappear but the fourth, larger than the others, is the functional mega- 

 spore. C, the enlarged megaspore, in which the nucleus has now divided into 

 two. D, a stage slightly later than C The two nuclei have migrated to opposite 

 ends of the young embryo-sac. E, a still later stage, with the embryo-sac much 

 increased in size. F, each of the two nuclei has divided into two. G, each of 

 these four nuclei has again divided into two, producing eight nuclei in all. H, 

 the embryo-sac, nearly mature. At the upper end (away from the micropyle of the 

 ovule) are three antipodal cells. At the lower end (toward the micropyle)are the 

 egg nucleus and the two synergids. The two endosperm nuclei in the middle 

 have not yet fused. I, fertilization. One small male nucleus is about to unite 

 with the endosperm nucleus (still double) and the other is about to fertilize the 

 the egg. {From A. I. Wcinstcin). 



separate carp(>ls; or a group of carpels which have become fused 

 together. Tn any case, the pollen is necessarily prevented from 



