PICKETT: EMBRYO-SAC OF ARISAEMA TRIPHYLLUM 231 
the division of a primary archesporial cell to form two or more 
megaspore mother cells or embryo-sac initials is not universal, 
or even regular, is proven by the presence of many ovules showing 
the development of but one tetrad of potential megaspores or 
but one embryo-sac, without evidence of other crushed or damaged 
initial cells (FIG. 17, 18, 22, 23, 24). Fig. 7, 9, 13 show cross 
sections of the nucellus with the megaspore mother cells variously 
arranged. 
THE TETRAD OF MEGASPORES 
As the inner integument grows and incloses the nucellus, and 
the outer integument appears, the megaspore mother cells increase 
in size, and the epidermal cells of the nucellus undergo periclinal 
division. This periclinal division, shown at its beginning in FIG. 
2 and 5, takes place more rapidly at the apex, sometimes forming 
a layer of four cells above the megaspore initials (FIG. 16, 18, 22). 
The division of lateral cells is also irregular and results in a final 
covering one to three layers of cells in thickness (FIG. 10, II, 24). 
During these changes in the size and form of the ovule, the mega- 
spore mother cells increase rapidly in size, becoming somewhat 
broader and much longer than the surrounding cells. Nuclear 
changes are also evident, and by the time the outer integument is 
well started most of the nuclei of the megaspore mother cells 
show either synapsis or closely related conditions preparatory to 
the first tetrad division (FIG. 7, 9, 12, 14, 16). The first division 
of the megaspore mother cell nucleus is quickly followed by the 
second, the result being a tetrad of potential megaspores (FIG. 18). 
The first division is usually followed by the formation of a cell 
wall as in FIG. 22 and 24, but this is not always the case, as 1s 
shown in FIG. 23. These divisions are usually transverse as shown 
in the cell at the left in F1G. 16, in the middle cell of Fic. 15a, and 
in FIG. 24. The resulting megaspores usually lie in a row parallel 
to the long axis of the nucellus (FIG. 18). That the first division 
may be longitudinal is shown by preparations similar to that in 
FIG. 10, and that the later divisions may vary from a trans- 
verse direction is shown in the upper cell in FIG. 22. From such 
divisions we might expect the formation of two parallel rows of two 
megaspores each, or a row of two parallel to the long axis of the 
nucellus with another transverse pair, as described for Symplo- 
