132 HERBERT SPENCER

division-products will be liberated as functional re-
productive cells or germ-cells, handing on the
tradition intact to the next generation.

An early isolation of the reproductive cells has been
observed in the harlequin fly (Chironomus) and in some
other insects, in the aberrant worm-type Sagitta, in
leeches, in thread-worms, in some Polyzoa, in some
small Crustaceans known as Cladocera, in the water-
flea Mo'ma, in some Arachnoids (Phalangidse), in the
bony fish Micrometrus aggregates, and in other cases.
In the development of the threadworm of the horse
according to Boveri, the very first cleavage of the
ovum establishes a distinction between somatic and
reproductive cells. One of the first two cells is the
ancestor of all the cells of the body; the other is the
ancestor of all the germ-cells. " Moreover, from the
outset the progenitor of the germ-cells differs from the
somatic cells not only in the greater size and richness of the
chromatin of its nucleus, but also in its mode of mitosis
(division), for in all those blastomeres (segmentation-
cells) destined to produce somatic cells a portion of the
chromatin is cast out into the cytoplasm, where it
degenerates, and only in the germ-cells is the sum-total of
the chromatin retained" (E. B. "Wilson, The Cell in
Development and Inheritance, 1896, p. III).

In the majority of cases, we admit, the reproductive
cells are not to be seen in early segregation, and the
continuous lineage from the fertilsed ovum cannot be
traced. In the majority of cases, the germ-cells are
seen as such after considerable differentiation has
gone on, and although they are linear descendants
of the ovum, their special lineage cannot be traced.
But it seems legitimate to argue from the clear casesture " which