HEREDITY 73 
At an early stage in the development of the embryo the 
future reproductive cells of the organism are often dis- 
tinguishable from those which are forming the body. 
These, the somatic cells, develop in manifold variety, and, 
as division of labour is established, they lose their likeness 
to the fertilised ovum of which they are the descendants. 
The future reproductive cells, on the other hand, are not 
implicated in the formation of the “body,” but, remaining 
virtually unchanged, continue the protoplasmic tradition. 
unaltered, and are thus able to start an offspring which 
will resemble the parent, because it is made of the same 
protoplasmic material, and develops under similar con- 
ditions. 
An early isolation of reproductive cells, directly con- 
tinuous and therefore presumably identical with the original 
ovum, has been observed in the development of some 
“worm types” —(Sagitéa, Thread-worms, Leeches, Polyzoa), 
and of some Arthropods (e.g. Aoima among Crustaceans, 
Chironomus among Insects, Phalangidee among Spiders), 
in Micrometrus aggregatus among Teleostean fishes, and 
with less distinctness in some other animals. A cell which 
will give rise to the germ-cells can be recognised in the 
gastrula stage of Cyclops, and in the very first segmentation 
stages of the thread-worm Ascaris. 
In many cases, however, the reproductive cells are not 
recognisable until a relatively late stage in development, 
after differentiation has made considerable progress. 
Weismann gets over this difficulty by supposing that the 
continuity is sustained by a specific nuclear substance— 
the germ-plasm—which remains unaltered in spite of the 
differentiation in the body. It is perhaps enough to say 
that, as all the cells are descendants of the fertilised ovum, 
the reproductive cells are those which retain intact the 
qualities of that fertilised ovum, and that this is the reason 
why they are able to develop into offspring like the 
parent. 
Finally, it may be noticed in connection with heredity, 
that there is great doubt to what extent the “body” can 
definitely influence its own reproductive cells. Animals 
acquire individual bodily peculiarities in the course of 
their life, as the result of what they do or refrain from 
