Theoretical objections to the biogenetic law had been raised from time
to time by many embryologists, but the positive testing of it by the
comparison of embryos in respect of the degree of development of their
different organs starts with Oppel's work of 1891.[528] He studied a large
number of embryos of different species at different stages of their
development, and determined the relative time of appearance of the
principal organs and their relative size. His results are summarised in
tabular form and have reference to all the more important organs. He was
led to ascribe a certain validity to the biogenetic law, but he drew
particular attention to the very considerable anomalies in the time of
appearance which are shown by many organs, anomalies which had been
classed by Haeckel under the name of heterochronies.

Oppel's main conclusions were as follows:--"There are found in the
developmental stages of different Vertebrates 'similar ontogenetic
series,' that is to say, Vertebrates show at definite stages
similarities with one another in the degree of development of the
different organs. Early stages resemble one another, so also do later
stages; equivalent stages of closely allied species resemble one
another, and older stages of lower animals resemble younger stages of
higher animals; young stages are more alike than old stages.... The
differences which these similar series show (for which reason they
cannot be regarded as identical) may be designated as temporal
disturbances in the degree of development of the separate organs or
organ-systems. Some organs show very considerable temporal dislocations,
others a moderate amount, others again an inconsiderable amount. Among
the developmental stages of various higher animals can be found some
which correspond to the ancestral forms and also to the lower types
which resemble these ancestral forms. On the basis of the tabulated data
here given there can be distinguished with certainty in the ontogeny of
Amniotes a pro-fish stage, a fish-stage, a land-animal stage, a
pro-amniote stage, and following on these a fully developed reptile,
bird or mammal stage."[529]

Oppel's methods were employed by Keibel[530] in his investigations on the
development of the pig, which formed the model for the well-known series
of _Normentafeln_ of the ontogeny of Vertebrates which were issued in
later years under Keibel's editorship. Keibel was more critical of the
biogenetic law than Oppe