REPRODUCTIVE CELLS. 123 



mother-cell of the embryo-sac (see Fig. 92, 1). This cell divides 

 into an upper or primary tapetal cell, and a lower larger 

 cell; the primary tapetal cell gives rise to three or more 

 secondary tapetal cells, which, later on, become obliterated by 

 pressure. 



The lower larger cell has cut off from its upper end a cell 

 which soon divides into two (so-called cap-cells), one of the 

 resulting cells dividing again; so that, finally, there are pre- 

 sent in a typical case seven cells (see Fig. 92, 2, 3, and 4), 

 viz. : — 



i. The three tapetal eells. 



ii. An intermediate tier of three cells, so-called cap-cells ; and 



iii. A large lower cell, which is the rudimentary embryo-sac ; the 



cap-cells (ii. ) become obliterated as well as the tapetal cells (i. ) by the 



pressure caused by further growth of the embryo-sac. * 



The young embryo-sac (macrospore) is a large cell possessing 

 cytoplasm and a relatively large nucleus. In the latter are, as a 

 rule, several nucleoli and an open chromatin reticulum. The 

 embryo-sac increases in size enormously, and soon comes to be 

 one of the largest cells present in the plant. 



Maturation of the macrospore consists in the occurrence of 

 certain changes in the cytoplasm and nucleus of this cell which 

 result in the production of the essential female cell or egg-cell, 

 and certain other accessory cells or nuclei, which will now be 

 described (see Plate iii.). The first change noticed is the division 

 of the nucleus of the embryo -sac into two by the mitotic method 

 of nuclear division ; another division then takes place in each of 

 these nuclei, so that there are now four nuclei present in the 

 cytoplasm, situated usually at the angles of regular figure (see 

 Plate iii., 4, and Fig. 93). A further division of each of these 

 four nuclei results in the production of eight nuclei, four of 

 which become massed together at the upper pole of the embryo- 

 sac and four at the lower pole. Of these eight nuclei, one from 

 each end passes to the middle of the embryo-sac, and these remain 

 for a time close together ; they are the so-called polar nuclei (see 

 Pig. 94), and in a short time they fuse to produce the definitive 

 nucleus (see Figs. 95, 96, and 97). At this stage there are then 

 present in the embryo-sac seven nuclei, three at each end and 



* See Goebel, Outline^: of Classification and Morphology. 



