248 W. HOFMEISTER ON THE DEVELOPMENT OF ZOSTEBA. 



The uppermost three cells of the row occupying the longitudinal 

 axis of the ovule are distinguishable from their neighbours, at a 

 very early period, by a larger size and greater concentration of 

 their contents ; this is particularly the case with the uppermost 

 of them, which gradually grows up to be the embryo-sac, while 

 the nucleus undergoes a profound change of structure by an active 

 multiplication of the cells, especially of those at its lower part. 



The cells of the surface of the upper end of the nucleus are 

 divided by longitudinal and transverse septa perpendicular to 

 the outer walls ; the same division is repeated in the " daughter- 

 cells/' The increase of this cellular hood, in length and compass, 

 thus keeps pace at first with that of the cell growing into an 

 embryo-sac (fig. 20), enclosed by it. A little later begins a very 

 rapid multiplication of the cells of the lower part of the nucleus, 

 in all three directions. In the first place the axial cells divide 

 by perpendicular longitudinal walls; the newly-formed cells 

 again divide by longitudinal walls standing at right angles to 

 those last produced. Thus from this time the embryo-sac no 

 longer rests upon a simple row of cells, forming the axis of the 

 mass of cells below it, but on a column formed of double pairs 

 of cells, which cells become repeatedly divided by horizontal 

 walls (fig. 21). The multiplication of the cells of the two peri- 

 pherical layers of the nucleus, only weak just below the growing 

 embryo-sac, becomes greater proceeding towards the base, where 

 the hitherto cylindrical mass of cells soon becomes strongly 

 bulged out. The multiplication of cells still continues in this 

 region even after the ovule has attained its normal size, so that 

 at the epoch of fertilization this part of the nucleus is composed 

 of cells one-fourth the size of those lying close beneath the 

 embryo-sac. Finally, the cells of the periphery of the nucleus 

 all divide once oftener by longitudinal and transverse walls 

 than those in contact with them ; they thus appear one-half the 

 height and breadth of the latter. 



During the further increase of size of the embryo-sac, the cells 

 surrounding it become gradually broken down and compressed. 

 The firmer, free, outer walls of the superficial cells are not attacked 

 by this softening and solution of the cellulose ; after the breaking 

 down of the inner tissue has advanced to a certain point, these 

 outer walls of the superficial cells form a homogeneous, connected 



