ONTOGENY is the development (embryonic and post-embryonic) of the
individual (on), or the science describing it.

PHYLOGENY is the development of the race or stem (phulon), or the
science describing it.

Roughly, ontogeny may be taken to mean embryology, and phylogeny what
we generally call evolution.

Further, the embryonic phenomena sometimes reproduce ancestral forms,
and they are then called PALINGENETIC (from palin = again): sometimes
they do not recall ancestral forms, but are later modifications due to
adaptation, and they are then called CENOGENETIC (from kenos = new or
foreign).

These terms are now widely used, but the reader of Haeckel must
understand them thoroughly.

The first five chapters are an easy account of the history of
embryology and evolution. The sixth and seventh give an equally clear
account of the sexual elements and the process of conception. But some
of the succeeding chapters must deal with embryonic processes so
unfamiliar, and pursue them through so wide a range of animals in a
brief space, that, in spite of the 200 illustrations, they will offer
difficulty to many a reader. As our aim is to secure, not a
superficial acquiescence in conclusions, but a fair comprehension of
the truths of science, we have retained these chapters. However, I
will give a brief and clear outline of the argument, so that the
reader with little leisure may realise their value.