EMBRYO DEVELOPMENT AND POLYEMBRYONY 
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separate (fig. 8), but are nevertheless entirely independent of each other in 
their further development, while the four primary embryos completely 
separate from each other, eight embryos being the normal product of each 
egg in Pinus.2 
Cleavage Polyembryony 
This separation of the zygote into a number of smaller units which 
undergo competition with each other is called cleavage polyembryony, to 
distinguish it from the simple polyembryony that may result from the 
fertilization of several eggs. The free nuclear divisions occurring during 
the proembryo stage in Finns are followed by the equal cleavages which 
organize the initial cells of each of the embryos that separate. It is well 
known that only one of these many embryos, produced either by cleavage 
polyembryony or by simple polyembryony survives to the maturity of the 
seed. This is surely a "survival of the fittest", if vigor in the embryo is any 
sort of measure of future fitness. 
This cleavage polyembryony with very definite proembryonic organiza- 
tion, which is present in Pinus, is apparently a primitive character in the 
development of this group of seed plants. This character tends to be 
modified or eliminated, reverting to the condition of simple polyembryony, 
as we advance along several phylogenetic lines, and is lost by the time the 
level of the Angiosperms is reached. If a splitting of the embryo is found 
among Angiosperms, which is rare, it is a cenogenetic character, a condition 
which has not necessarily been carried over from Gymnosperms. 
It is well known that the term polyembryony when applied to Angio- 
sperms has no such definite meaning as when applied to Gymnosperms, for 
among conifers the term has been used to designate the plurality of em- 
bryos which arise from the cleavage of one egg (cleavage polyembryony) , or 
from the fertilization of several eggs (simple polyembryony), but not other- 
wise. Among Angiosperms, as summarized by Coulter and Chamberlain 
(12), the embryos are known to arise from a plurality of eggs in only two 
species (widely separated in phylogeny), occasionally by a process of 
budding from the suspensor, and in only one species by the splitting of an 
embryo derived from the single egg. Furthermore, the extra embryos are 
derived from synergids in about twelve species, from nucellar or other tissue 
outside of the embryo sac in eleven or more species, from antipodal cells 
and even from endosperm in others, while false polyembryony may occur 
through the fusion of ovules, etc. Clearly this Angiosperm polyembryony 
has little in common with the phenomenon when found in Gymnosperms. 
In all Gymnosperms, polyembryony results in a selection of a single 
embryo from a larger number, long before the seed is matured, while in 
Angiosperms it is a common thing to find that several embryos survive 
to the maturity of the seed. The suspensor of the Gymnosperm. embryo 
2 A more detailed review and discussion of the proembryonic development of Abietineae 
is given in another paper by the author (4). 
