The descriptive statements of Haeckel’s ideas on the connection between ontogenesis and phylogeny must also be criticised:
It is not possible to place the course of an organism’s ontogenesis on the same level as its phylogeny. Haeckel’s assertion that all the form changes of an organism during its ontogenesis correspond to those of its own phylogeny could not be confirmed by comparative ontogenetic studies. It has been shown that in the ontogenesis of many organs, shifts in their temporal and spatial appearance (heterochrony, heterotopy) are not rare exceptions to their assumed phylogenetic appearance, but are the rule. The comparison of the ontogenesis of organisms that are phylogenetically closely related (e.g. amniota) also shows distinctive shifts, for example in the appearance of structures for heart or eyes, which would lead to absurd results if they were judged as recapitulation. Because of this situation, attempts are now mainly made to prove recapitulation only on an organ level.
In ontogenesis there is no appearance of features from adult phylogenetic forbears. It is not possible to place general homologous adult features of recent species or hypothetical phylogenetic forbears on a level with embryonic or foetal development. During the embryonic development of the foregut in reptiles and mammals, for example, which is phylogenetically said to be derived from the gill gut of fish and amphibians, at no time can the functional gills of supposed forbears with gill slits or gill lamellas be found.
There is an absence of objective guidelines for distinguishing between palingenesis and kenogenesis. It is not possible to label ontogenetic structures in themselves as palingenetic or kenogenetic (see above). For the “decision” on whether a certain developmental stage is palingenetic or kenogenetic can only be made precisely if the phylogeny is known” (Siewing 1987, p. 271). However, this prerequisite does not exist (compare Biogenetic Law – Examples).
The path of ontogenesis does not lead from “general to specific”. When comparing the earlier, “more primitive” ontogenetic stages of different organisms, there should be proof of an increasing similarity of germ cells. An individual’s ontogenesis would then proceed through various stages where first simple or general forms of organs and structures would be seen and later more specifically constructed forms, as in phylogeny.
This theory, too, has been refuted. When comparing increasingly earlier ontogenetic developmental stages of vertebrates, an increasing similarity of germ cells simply does not exist. The cleavage types of the fertilized ovum, the blastula or development of the blastocyst, gastrulation and development of the mesoderm as well as neurulation show distinct differences in amphibians, reptiles, birds and mammals (Fig. 270-272).