1918] BUCHHOLZ—PINUS 215 
initial is organized; the stage previous to this is a free nuclear 
division which organizes these several equivalent cells. Thus the 
proembryo stage in Pinus is the stage in which the divisions occur 
that bring about cleavage polyembryony. 
PROEMBRYO.—The free nuclear division which occurs after 
the 4 nuclei descend to the bottom of the egg is followed by 
walls which are complete for the lower tier of cells, but leave the 
upper tier in open communication with the egg. Thus, when the 
upper tier divides to form the rosette and the open tier above it, 
the cleavage is still essentially a free nuclear division. According 
to FERGUSON (13) and KILDAHL (19), this upper tier of the 8-celled 
proembryo usually divides before the lower, and in the resulting 
12-celled stage all of the cells with complete walls, namely, the 8 
cells of the lower 2 tiers, are embryo initials, which henceforth 
grow by means of an apical cell. This fact suggests the possi- 
bility that the upper open tier may also represent a tier of similar 
initials which has become abortive. This seems probable when we 
consider that in Pinus the lowest tier produces embryos immedi- 
ately; the rosette tier only after some delay, and then not always; 
while the upper open tier represents a group of initials that failed 
to organize. The presence of this upper abortive tier suggests a 
reduction from a more extensive form of polyembryony. 
The lower tier of the 8-celled proembryo sometimes divides 
before the upper one, according to Ki~paut (19), who also con- 
firms this order. No proof exists that the upper tier ever under- 
goes another division when the lower one divides first, and it is 
possible that the nuclei shown in her fig. 11 would have collapsed 
without undergoing further divisions. This would give us, in 
this case, only 8 embryo initials instead of 12 (counting the upper 
open tiers as potential embryo initials), of which only 4 function. 
It seems evident that this order of division is rather uncommon. 
According to the interpretation that the embryos are separately 
organized by means of initial cells in the proembryo, the latter 
stage has a new significance as a real preliminary stage in the 
embryogeny. However, the proembryo stage should be con- 
sidered closed in Pinus when the 12-celled stage is reached, rather 
than the 16-celled stage, and in the instance shown by KiLpAHL 
