Foundations of Embryology: Development and Evolutionary Theory

Core Concepts in Embryology

• Embryology: The branch of biology responsible for studying the development of an organism from a fertilized egg.
• Ontogeny: The changes that occur in an embryo during development. This has proven a powerful tool to determine the evolutionary relationships between different animals.
• Phylogeny: The study of the evolutionary history of any species of living thing.
• Oviparous: Organisms that reproduce by laying eggs.
• Viviparous: Individuals that develop within the mother's placenta.

Theories of Development

Theory of Preformationism

This theory alleged that a perfectly formed tiny body existed within the sperm or the egg. Embryonic development was considered simply an increase in the size of preformed parts of the adult. The main difference of opinion was whether the diminutive body was in the egg or the sperm. In 1699, Dalenpatius was the first to develop a theory on human reproduction; he observed human sperm under a microscope and believed he saw a man in miniature.

Theory of Epigenesis

This theory proposes that the structures of an adult organism were not preformed in the gametes. By the mid-nineteenth century, the concept of epigenesis was widely accepted by biologists as a result of careful studies. Scientists showed that only portions of the body formed during embryonic development.

Historical Milestones in Embryonic Development

• Aristotle: Made careful observations on development; however, due to the limited scientific thought of the time, few useful ideas were provided until the sixteenth century.
• 1592: D'Acquapendente Fabricius published the first detailed work on the fetal anatomy of chickens, fish, and mammals. His works are of great importance and he is considered the precursor of embryology.
• 1651: Harvey published papers on avian development and the early stages of development in mammals. He was convinced that the sperm fertilizes the whole body, but only the uterus can develop the fetus.
• 1672: Regnier de Graaf observed fluid-filled cavities in the ovaries of animals, which he identified as ovules.
• 1677: Anton van Leeuwenhoek observed how sperm and eggs from fish and amphibians united under his microscope. He believed that the sperm provided the vital substance for the development of the embryo.
• 1847: Albert von Kölliker stated that sperm are produced in the testicles and are responsible for fertilizing the egg.