Spermatophytes Angiosperms 275 



several megaspore mother cells may be found in Ranunculus; a 

 megasporangium with only one megaspore mother cell and only one 

 layer of cells surrounding it may be found in any of the Compositae. 

 Senecio aureus and Erectites hieracifolium are good and are particu- 

 larly easy to cut. In Trillium and in Cypripedium the embryo-sac is 

 formed from two megaspores, which are not separated by walls. In 

 Peperomia the megaspores are not separated by walls, and each 

 megaspore nucleus divides twice, so that a 16-nucleate sac is formed. 



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. 



In very young stages, before the appearance of the integument, 

 the ovary may be removed from the flower and placed directly in the 

 fixing agent, but in later stages, such as are shown in Fig. 100, strips 

 should be cut off from the sides of the ovary in order to secure more 

 rapid fixing and more perfect infiltration with paraffin. The dotted 

 lines in Fig. 99, C, show about how much should be cut off. This is 

 a much better plan than to secure rapid fixing and infiltration by 

 cutting the ovary into short pieces, because the ovules will be in 

 about the same stage of development throughout the ovary, and when 

 one finds desirable stages like those from which these photomicro- 

 graphs were taken, it is gratifying to have these pieces as long as 

 possible. 



Chromo-acetic acid, with the addition of a little osmic acid, is 

 good for fixing the entire series. Iron-haematoxylin, with a light 

 touch of orange, is best for the chromatin. For general beauty 

 and for the achromatic structures, the safranin, gentian-violet, 

 orange combination has not been excelled. The photomicrographs 

 (Figs. 96-98) illustrating the series from the archesporial cell (which, 

 in this case, is also the primary sporogenous cell and the megaspore 

 mother cell) to the four megaspore nuclei will repay a careful study. 



