272 Proceedings of the Royal Physical Society. 
fall to the ground. But still maintaining that the polar 
bodies serve to reduce the hereditary substance, he sets 
himself to answer the question, “ Why should the nuclear 
substance be doubled only to be halved again ?” 
His answer is as follows:—The germ-plasm, or active 
substance of the nuclear rods (or idants), may be thought 
of as built up of innumerable ancestral plasms or specific 
hereditary units (or “ids”), “each one of which, if it alone 
dominated the ovum, would be capable of guiding the whole 
ontogeny and of producing a complete individual of the 
species, but cannot be divided without losing this power.’ 
The significance of the longitudinal splitting of the idants, 
and the consequent doubling of their number, is an increase 
in the number of possible combinations. It lies in the 
attempt to bring about as intimate a mixture as possible of 
the hereditary units of both father and mother. “The 
significance of the reducing divisions in male and female 
germ-cells is a double one, first, the diminution of the ids by a 
half, and secondly, the arrangement of the idants in fresh 
combinations. The reducing division halves the number of 
idants so that one daughter-nucleus receives one set, and the 
other another set of ancestral plasms; it leads to a halving 
of the number of idants in such a manner that the most 
varied combinations can arise. Thus if there were twenty 
idants (and there are sometimes more), a process similar 
to that in Ascaris would yield 8,533,606 possible combina- 
tions. In short “the doubling of the idants before the 
‘reducing division’ renders possible an almost infinite 
number of different kinds of germ-plasm, so that every 
individual must be different from all the rest. And the 
meaning of this endless variety is to afford the material for 
the operation of natural selection.” 
(b) Parthenogenetic Reproduction.—In eighteen species of par- 
thenogenetic animals, viz., in eight Daphnids, a Branchiopod, 
two Ostracods, three Rotifers, and four Insects, it has been 
found that in the maturation of the ovum only one polar 
division occurs. 
Whether the single polar division is an ordinary “equal 
division,” dividing the collective ancestral plasms into two 
