Comparative embryology

Comparative embryology is the branch of embryology that compares and contrasts embryos of different species. It is used to show how all animals are related. Many things are compared (such as whether or not the organism has a or gill arches). Many components go into comparative embryology, and much information about the developmental similarities between species can be taken from its study, from which many conclusions can be drawn.

The first known person to study embryos was Aristotle. He observed different embryos and studied how different animals, who develop in different ways—by egg (oviparously) or by live birth (viviparously)—develop differently. He discovered that there were two main ways that the egg cell divides: holoblastic, where the whole egg divides and becomes the creature, and meroblastic, where only part of the egg becomes the creature. This was the furthest comparative embryology came until the invention of the microscope. After Aristotle, many people from Ernst Haeckel to Charles Darwin have contributed to the field of comparative embryology. They all noted how different embryos change during development.

In early years of comparative embryology, many erroneous theories had surfaced. One theory by Ernst Haeckel, a German biologist and philosopher, stated that all organisms go through a "re-run" of evolution while in development. He believed that to get to a mammal, an embryo first had to be a single-celled organism, then evolve into a fish, then an amphibian, then a reptile, then a bird, and finally reach a mammal. While this theory was widely accepted as the norm, it was disproved many years later.

The goal of comparative embryology is to make sense of how an embryo develops, and of how all animals are related. Comparative embryology also supports evolutionary theory, in the sense that all vertebrates develop similarly. The conclusion is that all vertebrates must have a common ancestor.

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