331 
which for varions reasons seems improbable. There is therefore 
much reason for thinking that segregation can occur before 
gameto-genesis begins, but there is no indication as to which 
are the critical divisions. 
Now that we may regard the formation of four cells of 
composition AB, Ab, aB, ab, as the foundation both of the 
coupling — and of the repulsion — series the problem in mani- 
festly somewhat simphfied. The time, excluding gameto-genesis, 
at which we can most readily imagine four such delinite quadrants 
to be formed is during the delimitation of the embryonic tissues. 
It is then that the plant is most clearly a single geometrical 
System. Moreover the excess of gametes of parental composition 
characterizing the coupling — and repulsion — series must certainly 
mean that the position of the planes of division by which the four 
quadrants are constituted is determined with regard to the 
gametes taking part in fertilisation. Though the relative positions 
of the constituents of the cells may perhaps be maintained 
throughout the history ot the tissues, it is easier to suppose that 
the original planes of embryonic division are determined according 
to those positions than that their influence can operate after 
complex somatic differentiation has been brought about. 
At some early stage in the embryonic development or per- 
haps in later apical divisions we can suppose that the n — 1 cells 
of the parental Constitution are formed by successive periclinal 
and anticlinal divisions of the original quadrants which occupy 
corresponding positions. The accompanying diagram gives a 
schematic representation of the process as we imagine it. (Fig. 3.) 
Whatever hypothesis be assumed the following points remain 
for consideration. 
1. We are as yet unable to imagine any simple System by 
which the four original quadrants can be formed by two s i m i 1 a r 
divisions. Evidently there must be two cell-divisions, and if in 
one of them we suppose AB to separate from ab, we cannot 
then represent the formation of Ab and aB. 
Therefore we are almost compelled to suppose that the 
original zygotic cell forms two similar halves, eaeh AaBb, and 
that the next division passes differently throngh each of these 
two halves, in the one half separatiug AB trom ab, aiul in the 
other half separating Ab from aB. The axes of this system mny 
well be determiAed by the position of the constituent parental 
