As has been agreed already, Haeckel's drawings glossed over the many differences in early embryo development. But this does not mean the drawings are complete fabrications, since the similarities are real, as has been confirmed by modern embryological studies. These similarities include the post-anal tail and the pharyngeal arches, as pointed out by BeagleBob, which give rise to the gills in fish, but in terrestrial vertebrates become various structures of the oropharynx, larynx and inner ear.
I also found
this article published last year showing conserved patterns of gene expression during embryo development around the time of appearance of the pharyngeal arches and somites, again suggesting a phylotypic stage in development. It is interesting that the gene expression at this time follows taxonomic groupings i.e. bilaterian, chordate, tetrapod, and amniote.
And just to reiterate what others have pointed out, the phylotypic stage is not the same as the biogenetic law.
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