STUDIES OF THE DEVELOPMENT OF PEPEROMIA HISPIDULA 367 
The tetrad stage was the next stage of the development of the em- 
bryo sac of which an adequate number of examples were seen for deter- 
mining the further history of these nuclei. In the young tetrads two 
spindles are seen usually at right angles to each other, and a well- 
marked cell plate is present on each (fig. 69). The nucleus at this 
time is 9 or 10 jj, in diameter; the chromatin net is rather open and of 
slender threads with many small granules. The nucleolus is again 
evident now, for the first time since late synapsis, as a rounded, darkly 
staining body, about 3 m in diameter (fig. 69). In many slightly older 
sacs the cell plates have been replaced by complete, though delicate, 
cell walls. The cytoplasm is thus cut to a tetrahedrally arranged 
group of cells, with (evidently), haploid nuclei (figs. 70, 71, which 
show adjoining sections of the same tetrad). This group has the 
characteristic structure, and, in the many features noted, the complete 
cytological history of a spore tetrad. It seems therefore impossible to 
escape the conclusion that this is a tetrad of megaspores, directly com- 
parable with the tetrad of microspores formed in the anther of this 
Peperomia and with the linear row of four megaspores formed in many 
angiosperms. (See Johnson, 1907, p. i; Coulter, 1908, pp. 363-4; 
Brown, 1908, p. 453). In only a few of the many cases observed were 
the four nuclei and the resulting cells arranged more nearly in a row 
(fig. 72). The question arises here whether this tetrad of megaspores 
is a primitive, fern-like feature, retained by this Peperomia. If so 
from what ancestor may it have been derived? On the other hand, if 
it is secondary in origin to what influence may this return from the 
serially arranged megaspores characteristic of nearly all seed plants to 
the tetrad arrangement found in pteridophytes be attributed? The 
consideration of these questions may be taken up later on, in our 
general discussion of results. 
The walls separating the young megaspores are very delicate, so 
delicate in fact that one sees at first only a cleavage plane, made more 
evident by a slight shrinkage (fig. 70, 71). The wall itself can, how- 
ever, usually be discovered in these cases, as a dark line next to one 
cytoplast or the other. At a slightly later stage of development these 
delicate tetrad, or spore walls have disappeared and the cytoplasm 
once more becomes a continuous mass throughout the embryo sac 
(figs. 73, 74). That is the protoplasts of all four megaspores fuse to 
form one composite mass of continuous cytoplasm, enclosing the four 
megaspore nuclei (see Lloyd, 1902; Brown, 1908, p. 449; Fisher, 
