The muscular system of the embryo, owing to its lack of union in the ventral wall, corresponds to the muscles of the shelled, headless mollusks, whose mantle is open in the same region. Meckel compares the bones of the higher vertebrates with the simpler bones of the lower forms, and even with the cartilages of the cephalopod. He points out that in the early human embryo the nerve cord extends the whole length of the spinal canal. He compares the simple heart of the embryo with that of worms, and a later stage, when two chambers are present, with that of the gasteropod mollusk. The circulation of the blood in the placenta recalls, he says, the circulation in the skin of the lower animals. The lobulated form of the kidney in the human embryo is compared with the adult condition in the fishes and amphibians. The internal position of the reproductive organs in the higher mammals recalls the permanent position of these organs in the lower animals. The posterior end of the body of the human embryo extends backwards as a tail which later disappears.
Some of these comparisons of Meckel sound very absurd to us nowadays, especially his comparison between the embryos of the higher vertebrates, and the adults of worms, crustaceans, spiders, snails, bivalve mollusks, cephalopods, etc. On the other hand, many of these comparisons are the same as those that are to be found in modern text-books on embryology; and we may do well to ask ourselves whether these may not sound equally absurd a hundred years hence. Why do some of Meckel’s comparisons seem so naïve, while others have a distinctly modern flavor? In a word, can we justify the present belief of some embryologists that the embryos of higher forms repeat the adult stages of lower members of the same group? It is important to observe that up to this time the comparison had always been made between the embryo of the higher form and the adult forms of existing lower animals. The theory of evolution had, so far, had no influence on the interpretation that was later given to this resemblance.
Von Baer opposed the theory of recapitulation that had become current when he wrote in 1828. According to Von Baer, the more nearly related two animals are, or rather the more nearly similar two forms are (since Von Baer did not accept the idea of evolution), the more nearly alike is their development, and so much longer in their development do they follow in the same path. For example two similar species of pigeons will follow the same method of development up to almost the last stage of their formation. The embryos of these two forms will be practically identical until each produces the special characters of its own species. On the other hand two animals belonging to different families of the same phylum will have only the earlier stages in common. Thus, a bird and a mammal will have the first stages similar, or identical, and then diverge, the mammal adding the higher characters of its group. The resemblance is between corresponding embryonic stages and not between the embryo of the mammal and the adult form of a lower group.