100 Proceedings of Royal Society of Edinburgh. [sess. 
(4) The sex-cells of the offspring being thus continuous with the 
parental sex-cells which give rise to itself, they will in turn 
develop into similar products. 
Now this fact of continuity of reproductive elements is obviously 
most satisfactory. If a fertilised egg-cell has "certain characters, 
x, y , z, it develops into an organism in which these characters x , y , z 
are expressed ; but, at the same time, the future reproductive cells 
are early set apart, retaining the characters x, y, z in all their 
entirety, to start a new organism again with the same capital. Bal- 
biani, who was not influenced by theoretical considerations, observed 
in Chironomus that the future reproductive cells were isolated before 
even the blastoderm was completed; that is to say, before any 
differentiation almost had occurred, a portion of the unadulterated 
ovum was insulated to continue the constancy of the species. 
In this aspect the reproductive cells form a continuous chain,, 
and the reproduction of like is as natural and necessary as it was in 
the Protozoa. Ho special theory is required. Similar conditions 
produce similar results. Unfortunately, however, a serious difficulty 
besets this easy theory. Such an early appearance and insula- 
tion of the reproductive cells, continuous with the very ovum itself, 
does indeed occur, and where it does the problem of heredity is 
simple, Early origin of special germ-cells, distinguished from those 
of the general “ body,” has been observed in some “ worm-types ” 
(leeches, Sagitta, threadworms, many Polyzoa) and in some Arthro- 
pods (Moina among crustaceans, not a few insects, Phalangidse 
among spiders), while indications of the same early separation are 
not wanting in a number of other organisms. But it must be dis- 
tinctly allowed that in most cases it is only after differentiation is 
relatively advanced that the future reproductive cells make their 
appearance. Thus we have to pass from the few cases as yet known 
of the continuity of the germinal cells, to the more' general, but less 
luminous, fact of the “ continuity of the germ-plasma.” 
Weismann’s Theory. — Weismann, like the previous investigators, 
had reached his conclusion independently. In the fact of con- 
tinuity between the reproductive elements of generations, the 
solution of likeness must be found. But a direct chain of cellular 
continuity can only be said to exist in a few cases. The solution 
which is proposed for the majority of cases is as follows : — - 
