The conservation of the phylotypic stage

Von Baer discovered the phylotypic stage of vertebrates at a time when earlier developmental steps were still unknown, and concluded therefore that the very first period of development was devoted to building the body plan. From this he derived the idea that embryonic development proceeds “from the general to the particular”. First it is the characteristics of the phylum that appear, then come the features that specify the class, the order, the family, the genus and the species, and only at this point the signs appear that distinguish an individual animal from others.
Within a few decades, however, embryologists discovered that the phylotypic stage is preceeded by a number of developmental steps, and that differences between the embryos of the same phylum are clearly visible even before the phylotypic stage and not only afterwards (Figura 7-3). The striking similarity which is observed at the phylotypic stage is therefore the result of two opposite patterns of development: before the phylotypic stage, the morphological differences between the embryos of a phylum are decreasing, whereas after that stage they are increasing. The tendency to move “from the general to the particular” was valid after the phylotypic stage, but not before it, and this deprived the idea of the power to explain the whole of development, the very point that von Baer had insisted on.
After the publication of Darwin’s Origin of Species, the entire approach to develoment changed radically, and the idealistic principle proposed by von Baer was replaced by an evolutionary interpretation. The experimental pattern that was expected in this new framework, however, did not change: the developmental stages of embryos are a result of evolution, and since chance variations increase the diversity of organisms, we should observe that differences within a phylum increase in all stages of development, and not only after the phylotypic stage. The pattern which is observed before the phylotypic stage, simply cannot be explained by the same evolutionary mechanism that produced the opposite pattern of the other stages, and this is a problem which has never been given a satisfactory answer. Haeckel, for example, claimed that pre-phylotypic stages were simply a result of secondary complications.