April 1892.] THE BOTANICAL SOCIETY OF EDINBURGH. 369 



As the two primordial cells, whom hunger has driven 

 to make acquaintance, cannot get any or only little nourish- 

 ment from either the antipodal or the micropylar ends of 

 the sac, they would have to degenerate if it were not for the 

 presence of the large vacuole in the centre of the sac. This 

 vacuole exerts pressure on the surrounding nucellar cells, 

 enfeebles them and causes their degeneration ; a degeneration 

 which supplies the primordial cells with nutriment. It has 

 already been pointed out how this degeneration proceeds 

 from above downwards, therefore most nourishment will be 

 flowing into the micropylar end of the sac. This fact may 

 explain why the nucleus of the antipodal primordial cell 

 travels upwards into the micropylar half of the sac. 



This history of the primordial cells gives us a clue as to 

 the position of the vacuole in the egg-cell : — As the latter 

 does not occupy the very apex of the embryo-sac, a position 

 held by the synergidte, and as further the growth of the 

 embryo-sac during its early development is chiefly a growth 

 in length, taking place at the very apex, the synergidse will 

 consume all available nutriment coming from the degenerating 

 cells and leave the egg to its own resources. We have 

 further seen how the two primordial cells were the most 

 starved, and how through degeneration of the periblem-cells 

 they were supplied with nutriment ; as now the micropylar 

 primordial cell lies in close contact with the ovum (figs. 26, 

 28, &c.), the latter will receive its food-supply from the 

 degenerating periblem-cells through the medium of the 

 protoplasm of the micropylar primordial cell. Fig. 28 

 illustrates how the protoplasm of the primordial cell {m. p. c.) 

 forms a thick layer just beneath the ovum {ov), its sister- 

 cell. As in the synergida?, so here the nucleus and the bulk 

 of the protoplasm tend to occupy that side of the cell through 

 which nutriment is flowins in. 



The three antipodes may be arranged in various ways 

 according to the breadth of the basal end of the embryo-sac. 

 To find all three placed side by side, with their long axes 

 parallel to the long axis of the sac is common (figs, 25", 25'', 

 2%, 27), yet one cell is usually at a higher level than the two 

 others, and this higher cell is the sister-cell of the antipodal 

 primordial cell ; it may even (fig. 29^) be placed above one 

 of the other two antipodes. 



