116 DEVELOPMENT OF OVULE [CH. 



or more minute humps of embryonic tissue arise either 

 on its surface, usually at or near its margin, or from 

 the axis at its base, and develope into the ovule or 

 ovules. 



Taking the commonest case as the type, this em- 

 bryonic emergence is seen to start by the bulging up of 

 the tissue of the placenta, owing to increased growth and 

 cell-division of a few sub-epidermal cells, and rapidly 

 assumes the form of an egg-shaped protuberance the long 

 axis of which curves over as it rises. This lump is 

 composed entirely of embryonic tissue, but at an early 

 stage two signs of differentiation appear in it : certain 

 external cells in a zone below the dome-shaped apex 

 protrude by more rapid division and raise a rim which 

 proceeds to stand out from the dome, at first like an 

 egg-cup round an egg; while a cell just beneath the 

 epidermis at the apex of the dome becomes remarkable 

 for its size and the brilliancy of its protoplasm and nucleus. 

 This latter is the spore-forming tissue (archespore), 

 while the rim of investing cells is the inner integu- 

 ment of the ovule ; the egg-shaped protuberance which 

 gives rise to both being the nucellus of the ovule. 



As the nucellus rapidly enlarges, by the growth and 

 division of its cells, a second rim-like investing layer may 

 arise outside the first and proceed to overlap closely the 

 first integument as a second or outer one. 



Meanwhile, the spore- forming tissue divides into two 

 cells by a wall at right-angles to its long axis, and the 

 lower of the two cells soon divides again by one or two 

 walls parallel to the first one, and we thus have a row 

 of three or four cells in the dome of the nucellus. Of 

 these cells the upper ones soon degenerate and are 

 absorbed by the lower one, which is the mother-cell of 

 the embryo-sac. 



