XIII] DEVELOPMENT OF OVULE 117 



Meanwhile, the curvature of the nucellus continuing, 

 the two integuments close up over the dome and nearly 

 meet there, but leave a narrow passage the micropyle 

 between them. 



While these processes are approaching completion the 

 embryo-sac mother-cell undergoes a division by a wall 

 across its middle, and each of the two cells thus formed 

 usually divides again by a parallel wall ; so that we now 

 find the axial row of cells of the nucellus terminated 

 by four cells, capped above by the degenerating cells 

 referred to above, which are rapidly undergoing disintegra- 

 tion, trapped as they are between these four cells and the 

 epidermis. 



Next, the lowermost of the four cells, developed from 

 the embryo-sac mother-cell, begins to enlarge at the 

 expense of the other three, as well as at that of embryonic 

 cells of the nucellus in the immediate neighbourhood, and 

 becomes the embryo-sac. 



At this stage, therefore, the typical anatropous ovule 

 shows the following features. 



The nucellus, invested by two integuments, between 

 which, at the organic apex, is the micropyle, is turned 

 sharply back on its attachment; a sort of broad stalk, 

 known as the raphe, and up which a vascular-bundle 

 runs in connection with the bundles of the carpel, lying 

 closely on it. This bundle ends at the organic base of 

 the nucellus termed the chalaza. Inside the nucellus, 

 towards the micropylar end, is the young embryo-sac filled 

 with a mass of nucleated protoplasm, and pressing, as it 

 enlarges, not only on the cap-cells and tapetum above, but 

 also on the surrounding cells of the nucellus. 



The embryo-sac now begins to enlarge considerably, 

 evidently at the expense of the cap-cells and tapetum, 

 as well as at that of the surrounding nucellar tissue, for 



