The comparative embryology is a branch of embryology that focuses on contrasting the development patterns in different embryos. This discipline has its origins in ancient times, beginning to take shape in the minds of thinkers like Aristotle. Later, with the invention of the microscope and appropriate staining techniques, it began to grow as a science..
When we speak of comparative embryology, it is inevitable to evoke the famous phrase: ontogeny recapitulates phylogeny. However, this statement does not accurately describe current principles of comparative embryology and has been discarded..
Embryos resemble other embryonic forms of related species, and do not resemble adult forms of other species. That is, a mammalian embryo is not similar to an adult fish, it is similar to a fish embryo.
Comparative embryology has been used as evidence of the evolutionary process. The obvious homologies that we observe in the development of similar groups would be totally unnecessary if an organism were not a modification of the ontogeny of its ancestor..
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The first study focused on comparative embryology dates back to the time of Aristotle, in the 4th century BC..
This philosopher and scientist described the different possibilities of births among animal species, classifying them as oviparous, if they laid eggs, viviparous, if the fetus was born alive, or ovoviviparity, when the production of an egg that opens inside the body occurs..
Furthermore, Aristotle is also credited with identifying the holoblastic and meroblastic segmentation patterns. The first refers to the whole egg that divides into smaller cells, while in the meroblastic pattern only a part of the egg cell is destined to be an embryo, and the remaining portion is the yolk..
Embryological studies were practically non-existent for more than two thousand years, until William Harvey in 1651 announced his motto ex ovo omnia (all from the egg), concluding that all animals originate from one egg cell.
After the invention of the microscope, embryology takes on a new nuance. In 1672, the researcher Marcello Malpighi investigated the development of the chicken embryo, using this new optical technology.
Malpighi first identified the neural groove, the somites responsible for muscle formation, and observed the circulation of the veins and arteries connected to the yolk sac.
Over the years and the invention of the most modern staining techniques, embryology began to grow by leaps and bounds. Pander is credited with discovering the three germ layers using chicken embryos: the ectoderm, endoderm, and mesoderm..
Rathke looked at the embryos of different animal lineages, and concluded that the embryos of frogs, salamanders, fish, birds and mammals presented incredible similarities..
In more than 40 years of research, Rathke identified the pharyngeal arches and their fate: in fish they form the gill apparatus, while in mammals they form the jaw and ears..
In addition, he described the formation of a series of organs. He also studied the embryological process in some invertebrates.
An iconic phrase in comparative embryology is: "ontogeny recapitulates phylogeny." This expression seeks to summarize the theory of recapitulation, associated with Ernst Haeckel. Recapitulation ruled embryology during the 19th century and part of the 20th century.
According to this theory, the developmental stages of an organism are reminiscent of its phylogenetic history. In other words, each stage of development corresponds to an ancestral evolutionary stage..
The appearance of gill-like structures in mammalian embryos is one of the facts that seems to support the recapitulation, since we assume that the mammalian lineage originated from an organism similar to today's fish..
For the advocates of recapitulation, evolution works by adding successive states at the end of development.
However, for current evolutionary biologists it is clear that evolution does not always work by adding terminal states and there are other processes that allow us to explain morphological changes. For this reason, biologists accept a broader vision and this phrase has already been discarded..
Karl Ernst von Baer gave a much more satisfactory explanation of the similarities of embryos, challenging what Ernst Haeckel proposed..
One of his most outstanding contributions was to point out that the most inclusive characteristics of a taxon appear in ontogeny before the more specific characteristics - proper to order or class, for example..
While von Baer was conducting his research in comparative embryology, he forgot to label two embryos. Although he was a scientist with a trained eye, he was unable to distinguish the identity of his samples. According to von Baer "they could be lizards, small birds or even mammals".
Thus, the literature usually groups the main conclusions of this researcher into four postulates or principles, as follows:
If we compare two vertebrate embryos, we will see that the first characteristics that appear are those related to "being a vertebrate."
As development progresses, specific characteristics emerge. All vertebrate embryos have notochord, branchial arches, spinal cord, and a particular type of ancestral kidney. And then the specific ones: hair, nails, scales, etc..
For example, when development is incipient, all vertebrates have similar skin. Later the scales appear in fish and reptiles, feathers in birds or hair in mammals..
The famous gills of embryonic mammals do not resemble the gill slits of adult fish. In contrast, they resemble the clefts of the fish embryo.
Human embryos will never go through a state reminiscent of a fish or a bird in its adult form. They will be similar to fish and bird embryos. Although this statement is similar to the third, it usually appears as an additional principle in the literature..
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