esses which appear subsequently through _adaptation_
to the needs of embryonic or larval life, and accordingly can _not_ be
regarded as repeating the organisation of an earlier independent
ancestral form, can clearly have for the understanding of the ancestral
history only a quite subordinate and _secondary_ importance.

"The first I have named _palingenetic_, the second _cenogenetic_.
Considered from this critical standpoint, the whole of ontogeny falls
into two main parts:--First, _palingenesis_, or 'epitomised history'
(_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit
history' (_Faelschungsgeschichte_). The first is the true ontogenetic
epitome or short recapitulation of past evolutionary history; the second
is the exact contrary, a new foreign ingredient, a falsifying or
concealing of the epitome of phylogeny."[379]

As examples of palingenetic processes in the development of Amniotes,
for instance, may be quoted the separation of two primary germ-layers,
the formation of a simple notochord between medullary tube and
alimentary canal, the appearance of a simple cartilaginous cranium, of
the gill-arches and their vessels, of the primitive kidneys, the
primitive tubular heart, the paired aortae and the cardinal veins, the
hermaphroditic rudiment of the gonads, and so on. Cenogenetic processes,
on the other hand, include such phenomena as the formation of yolk and
the embryonic membranes, the temporary allantoic circulation, the navel,
the curved and contracted shape of the embryo, and the like.

The most important phenomena to be included under the general heading of
cenogenesis are, first, the occurrence of food-yolk, and second, those
anomalies of development which are classed by Haeckel as heterochronies
and heterotopies.

It is to the influence of the different amounts of yolk present in the
egg that are due the great differences in the segmentation and
gastrulation processes, which almost mask their true significance.

Heterochronic processes are such as arise through the dislocation of the
proper phylogenetic order of succession: heterotopic processes in the
same way are caused by a wandering of cells from one germ-layer to
another. The two classes of phenomena are disturbances either of the
proper spatial or of the proper temporal relation of the parts during
development.

Heterochrony shows itself, as a rule, either as an acceleration or as a
retardation of developmental events, as compar