Embryo of Sequoia sempervirens. 21 
Conifers the fusion-nucleus gives rise to a number of free nuclei, which 
take up a position in the plane at the base of the oospore. These free 
nuclei, by dividing, give rise to three tiers of cells with complete walls. 
The uppermost of the three tiers remains in the oospore, the middle tier 
develops the suspensors, and the lowest tier the embryo. This is no doubt 
true for Pinns and many other Conifers, but in Sequoia the embryo de- 
velops in quite another way. 
Soon after the daughter-nuclei of the first cleavage have been organ- 
ized, a cell-plate is formed between them, and this results in the develop- 
ment of two distinct cells, each surrounded by its own cell-wall. The 
first cleavage, therefore, does not result in the formation of free nuclei. 
Fig. 33 shows the two daughter-nuclei of the first cleavage. It also shows 
the cell-plate developing on the kinoplasmic fibrils which extend between 
the nuclei. The two cells thus organized from the first division occupy 
almost the entire cavity of the oospore, and lie one behind the other. 
These two cells now divide in the same plane as the first division, and 
the pro-embryo now consists of a single row of four large cells, as shown 
in Fig. 34. An examination of this figure makes it apparent that the 
division of the first two cells of the pro-embryo is nearly or quite simul- 
taneous. In this connexion it is interesting to note that Arnoldi reports 
the presence of but three cells in the embryo at this time. He states that 
the uppermost cell of the first cleavage functions no further, but that the 
lower one divides and organizes two cells which become the suspensor and 
the embryo proper respectively. As shown in Fig. 34, there can be no 
doubt as to the division of both daughter-nuclei of the first cleavage. 
The next stage observed showed five cells in the oospore, and, judging 
from the position of them, the fifth one arises by a division of the lowest 
cell in the row of four. The five cells take up a position as shown in 
Fig- 35- The fifth or lowest cell in the row now enlarges and divides. 
Of the last two cells thus organized, the end one becomes the embryo 
proper and the other one develops into the suspensor. As soon as the 
embryo-cell and the suspensor-cell are organized the latter enlarges very 
rapidly and becomes very much elongated. As it increases in length it 
bends downward and carries the embryo-cell, at its apex, down through 
the endosperm. As the embryo forces its way through the prothallial 
tissue the cells of the latter show every sign of disorganization, and are 
no doubt absorbed by the developing embryo. Fig. 37 shows a suspensor 
with its large single nucleus and a two-celled embryo at its apex. 
Summary. 
For some time after the separation of the tetrads, the microspore 
contains a single nucleus. Before leaving the sporangium this nucleus 
divides, so that at the time of pollination there are two nuclei present. 
