n of his "principle of connections."

Between comparative anatomy and embryology there exists a close
connection, for the one throws light on the other. "While in some cases
the same organ shows only slight modifications in its development from
its early beginnings to its perfect state, in other cases the organ is
subjected to manifold modifications before it reaches its definitive
form; we see parts appear in it which later disappear, we observe
alterations in it in all its anatomical relations, alterations which may
even affect its texture. This fact is of great importance, for those
changes which an organ undergoes during its individual development lead
through states which the organ in other cases permanently shows, or at
the least the first appearance of the organ is the equivalent of a
permanent state in another organism. If then the fully developed organ
is in any special case so greatly modified that its proper relation to
some organ-series is obscured, this relation may be cleared up by a
knowledge of the organ's development. The earlier state indicated in
this way enables one to find with ease the proper place for the organ
and so insert it into an already known series. The relations which we
observe in an organ-seriation are then the equivalent of processes which
in certain cases take place in a similar manner during the individual
development of an organ. Embryology enters therefore into the closest
connection with comparative anatomy.... It teaches us to know organs in
their earliest states, and connects them up with the permanent states of
others, whereby they fill up the gaps which we meet with in the various
series formed by the fully developed organs of the body" (pp. 6-7).

This recognition of the parallelism between comparative anatomy and
embryology is, of course, the kernel of the Meckel-Serres law. For
Gegenbaur it had a very definite evolutionary meaning--he subscribed to
the evolutionary form of it, the biogenetic law. How near his conception
of the relation between ontogeny and phylogeny came to the old
Meckel-Serres law may be gauged from the following passage, taken from a
later work:--"Ontogeny thus represents, to a certain degree,
palaeontological development abbreviated or epitomised. The stages which
are passed through by higher organisms in their ontogeny correspond to
stages which are maintained in others as the definitive organisation.
These embryonic stages may accordingly be explained