IS92.] Phenomena and Development of Fecundation. 299 



At the moment when the demiovocenters ami demispermo- 

 centers are on the point of uniting. the aureole rapidly disap- 

 pear and true aster become apparent with their perfectly dis- 

 tinct fibrils, much different from the radiations which are 

 visible till then (fig. 29). The demicenters unite and fuse to 

 form the first asterocenters. 



The author concludes that fecundation consists not only in 

 the addition of two nuclei arising from different individ- 

 uals of different sexes, but in the union of two demispenuo- 

 centers with two demi-ovocentres to form the first two astro- 

 centers. All succeeding astrocenters are derived in eqtial 

 parts from the mother and father. 



Fecundation in Higher Plants. 

 Development of Embryo Sac and Egg Apparatus :— -In the 

 higher plants (the anthophytes or spermophytes) we are par- 

 ticularly concerned with the embryo sac and its inclosed egg 

 apparatus. It is necessary that we should thoroughly under- 

 stand its development. The embryo sac first shows itself as 

 an enlarged specialized cell in the upper central part of the 

 nucellus or body of the ovule (fig. 30, a). In the maturation 

 the nucleus divides and the two daughter nuclei thus formed 

 travel in opposite directions, one going to the apex, the other 

 to the base of the embryo sac which has, in the meantime, been 

 growing larger and longer (fig. 31). After reaching their re- 

 spective ends each divide again (figs. 32 and 33) and the two 

 in each end thus formed again divide (fig. 34) forming a 

 tetrad of nuclei at both the apex and the base of the embryo 

 sac. Now a very peculiar thing happens. One of the nuclei 

 from each tetrad thus formed leaves its position and journeys 

 toward the centre of the embryo sac where they come together 

 and fuse, forming the nucleus proper of the embryo sac (fig. 

 35, c). There is now left at each end of the embryo sac 

 three nuclei of the original tetrad. The nuclei of the upper 

 end become partitioned off by walls and form the egg ap- 

 paratus proper. The two upper cells, the so-called synergidss 

 or accessory cells (fig. 35, a) are of doubtful function, being 

 merely of secondarv value in fertilization. They are some- 



