History of Embryology

Embryology has long been regarded as an important discipline for understanding the diversity of life forms. In the early 1800s, life was viewed as a continuous chain of increasing biological complexity, termed the chain of being. The development (or ontogeny) of an organism was in turn thought of as paralleling this chain of being, in that during its embryology, a higher organism passed through all of the adult stages of organisms found lower on the chain. This idea was termed the law of parallelism, and it dominated the field of embryology for the first part of the nineteenth century until it was refuted in 1828 by embryologist Karl von Baer.

Von Baer did not accept the notion that all of life formed as a single, continuous chain. He instead claimed that organisms are divided into four different groups, each of which shares similarities in its early development. Von Baer asserted that within a particular group, the more general features appear earlier in development than the more specialized features, and that the more specialized features develop from the more general ones. Using this concept, it would be impossible for an embryo to pass through adult forms of a lower animal, because development proceeds from the general to the specific, and not as a succession of specific adult forms. These ideas are today known as von Baer's laws (1828), and they remain valid as a framework for comparative studies in embryology. Following the publication of von Baer's laws, Charles Darwin addressed the importance of embryology for understanding evolution in his Origin of Species (1859), when he stated: "Community of embryonic structure reveals community of descent," which mimicked the ideas of von Baer in evolutionary terms.

Until the mid-nineteenth century, embryology was held to have a central role in revealing evidence for evolution, but its actual role

Figure 1

Illustration of Karl von Baer's Laws

Figure 1

Illustration of Karl von Baer's Laws

Source: Romanes, G. J. 1896. Darwin and after Darwin. Chicago: Open Court Publishing, figs. 57-58, pp. 152-153.

Note: Early vertebrate embryos are indistinguishable, but as development proceeds the embryos develop characteristics unique to their species

Source: Romanes, G. J. 1896. Darwin and after Darwin. Chicago: Open Court Publishing, figs. 57-58, pp. 152-153.

Note: Early vertebrate embryos are indistinguishable, but as development proceeds the embryos develop characteristics unique to their species in the evolutionary process remained vague. Ernst Haeckel (1866) was the first to actually suggest a mechanism for embryology in evolution; he asserted that ontogeny recapitulates phylogeny. This idea was referred to as the biogenetic law, and it was similar to the law of parallelism in that it proposed that during its development, an organism passes through all of the adult stages of lower forms. The major difference between the biogenetic law and the law of parallelism was that Haeckel cast his theory in evolutionary terms. He proposed that in evolution new features are added to the end of an organism's ontogeny, with the ear lier stages being either condensed or deleted. According to Haeckel, careful scrutiny of an organism's embryology could reveal evidence of its evolutionary history.

The biogenetic law fell out of fashion in the late 1800s, when embryology transformed from a field that was mainly descriptive to one that was primarily experimental. This transformation was led by the embryologist Willhelm Roux, who in 1894 introduced a new journal, Developmental Mechanics. With the establishment of this journal, a new standard was set for embryological studies. Roux and others claimed that embryology must be explained mechanistically, and that can be accomplished only through experiments and not through observation. Evolution was viewed by this new generation of embryologists as too speculative and therefore no longer important for the field of embryology.

Evolution and development would not become reunited again until the late twentieth century, under the auspices of a new field called evolutionary developmental biology. Evolutionary developmental biology seeks to connect the mechanisms of development to the diversity of life forms. This new synthesis is now possible because of the recent conceptual and technical advances in embryology (see Embryology Today, below).

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