the various foetal membranes and the corresponding changes in the
blood vessels. Further instances are: the dual structure of the heart
cavity, the temporary division of the plates of the primitive
vertebrae and lateral plates, the secondary closing of the ventral and
intestinal walls, the formation of the navel, and so on. All these and
many other phenomena are certainly not traceable to similar structures
in any earlier and completely-developed ancestral form, but have
arisen simply by adaptation to the peculiar conditions of embryonic
life (within the foetal membranes). In view of these facts, we may now
give the following more precise expression to our chief law of
biogeny: The evolution of the foetus (or ontogenesis) is a condensed
and abbreviated recapitulation of the evolution of the stem (or
phylogenesis); and this recapitulation is the more complete in
proportion as the original development (or palingenesis) is preserved
by a constant heredity; on the other hand, it becomes less complete in
proportion as a varying adaptation to new conditions increases the
disturbing factors in the development (or cenogenesis).

The cenogenetic alterations or distortions of the original
palingenetic course of development take the form, as a rule, of a
gradual displacement of the phenomena, which is slowly effected by
adaptation to the changed conditions of embryonic existence during the
course of thousands of years. This displacement may take place as
regards either the position or the time of a phenomenon.

The great importance and strict regularity of the time-variations in
embryology have been carefully studied recently by Ernest Mehnert, in
his Biomechanik (Jena, 1898). He contends that our biogenetic law has
not been impaired by the attacks of its opponents, and goes on to say:
"Scarcely any piece of knowledge has contributed so much to the