416 FERTILIZATION AND FORMATION OF FRUIT IN PHANEROGAMS. 



sets of tubes reach the ovary, or whether one set receives a check of some sort. In 

 fact we want to know whether foreign pollen is prepotent over own pollen (where 

 both are competent to fertilize), and if so how the prepotency is accomplished. This 

 and a host of similar problems await solution. 



Passing on now to the union of spermatoplasm and ooplasm it is first of all 

 necessary to describe the structure of the ovule in some detail. The egg-cell which 

 has to be fertilized forms but a small portion of the ovule. It is produced in 

 Flowering Plants within a large cell prominently developed and termed the Emihryo- 

 sac. This embryo-sac is one of the cells of the central portion of the ovule known 

 as the Nucellus, and this cell as the time of maturation of the ovule approaches 

 grows much in size, in part at the expense of its neighbours. Ultimately the 

 embryo-sac occupies a large portion of the nucellus, being still inclosed by a layer 

 of small nucellar cells. Outside this are the integuments. They are not completely 

 closed, but at one spot an opening (the micropyle) is left, the entrance by which 

 the pollen-tube gains access to the embryo-sac. The general relations of the embryo- 

 sac to the other portions of the ovule are shown in fig. 815^, a longitudinal section 

 of the ovule of Ornithogalum. In fig. 316 three stages of the embryo-sac of Mono- 

 tropa are shown just at the time of fertilization. At an earlier stage the embryo- 

 sac is a uni-nucleate cell, and before the arrival of the pollen-tube at the micropyle 

 its contents divide up into a number of small cells, which, though devoid of cell- 

 membranes, are readily distinguishable from one another. At the apical or micro- 

 pylar end three of these cells are situated. The two uppermost, side by side, are 

 known as the synergidce, whilst close below them and slightly to one side (cf. fig. 

 316) is the egg-cell, destined to be fertilized. These three cells constitute the " egg- 

 apparatus". At the other extremity of the embryo-sac, i.e. at the base, three cells 

 are present which are known as the antipodal cells. These, soon after their forma- 

 tion, develop walls around themselves and appear to play no part in subsequent 

 phenomena. Besides these, there are two nuclei (the so-called polar nuclei) lying 

 in the protoplasm of the embryo-sac, one close above the antipodals, the other just 

 below the egg-apparatus (fig. 316^). These two approach one another at about the 

 moment of fertilization and fuse (figs. 316" and 316^) about midway between egg- 

 apparatus and antipodals. They give rise ultimately to the food-material which 

 nourishes the young fertilized egg-cell during its early stages of development. 



The egg-cell and its attendant synergidae contain each a well-marked nucleus 

 and vacuoles. In the egg-cell the vacuole is above the nucleus (fig. 316 ^), in the 

 synergidae the vacuoles are below the nuclei (fig. 316^). The nucleus of the egg- 

 cell is often very large. The structure and changes of cell-nuclei have already been 

 briefly reviewed at vol. i. p. 581. 



Meanwhile, in the pollen-tube changes have also taken place. Actually in the 

 pollen-cell before the pollen-tube is produced two nuclei are present. Though both 

 of these enter the tube one is quite sterile and soon atrophies. The other, however, 

 surrounded by a small portion of protoplasm, but destitute of wall, constitutes the 

 male sexual cell. It is carried, embedded in the general protoplasm of the pollen- 



