SPERMATOPHYTES— ANGIOSPERMS 



363 



separated by walls. In Lilium, Tulipa, Fritillaria, Erythronium, and 

 many others, the embryo sac is formed by all four megaspores, which 

 are not separated by walls. In Peperomia, the Peneaceae, and some 

 species of Ewphorhia, the sac is formed by the four megaspores, not 

 separated by walls, and the sac has 16 free nuclei. In Plumbagella the 

 four megaspores, not separated by walls, constitute the mature sac. 









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Fig. 130. — Lilium philadelphicum: photomicrograph of transverse section of ovary showing, in 

 one of the ovules on the left, the first mitosis in the megaspore mother-cell; and, in one of the ovules 

 on the right, the second mitosis which gives rise to the four megaspore nuclei. Chromo-acetic acid; 

 safranin, gentian violet, orange. Cramer contrast plate; 16-mm. objective; ocular X4; yellowish- 

 green filter and also a strong filter such as is used in outdoor work; camera bellows, 30 cm. ; exposure, 

 2 minutes. Negative by Miss Ethel Thomas. X64. 



one of the megaspores functioning as the egg, two more fusing to form 

 the endosperm nucleus, while the fourth megaspore aborts; so that the 

 embryo sac, ready for fertilization, contains only two nuclei. 



The reduction of chromosomes takes place during the two mitoses 

 by which the mother-cell gives rise to four megaspores. The figures are 

 much larger than in the corresponding mitoses in spermatogenesis but 

 so much more tedious to secure that most studies in reduction have 

 been based upon divisions in the pollen mother-cell. Lilium is quite 

 favorable for a study of oogenesis, but it must be remembered that it 

 is exceptional in having an embryo sac formed from four megaspores. 



