Germ-Layer Theory: Embryonic Development of Multicellular Organisms

The Germ-Layer Theory of Development is a biological concept that explains how complex multicellular organisms develop from a single fertilized egg. According to this theory, during early embryonic development, the fertilized egg undergoes cell division and gives rise to three primary layers of cells, known as germ layers. These germ layers give rise to different types of tissues and organs in the adult organism. The Germ-Layer Theory of Development is considered one of the fundamental concepts in developmental biology and has been essential in understanding how organisms grow and develop.

 

Casper Friedrich Wolff first proposed the epigenetic theory of generation in 1759. According to his theory, each individual begins as an undifferentiated mass in the egg and gradually differentiates and grows after conception. Despite providing evidence to support his theory, Wolff's work was largely disregarded by the scientific community at the time. However, during the following century, his theory was revisited and served as the foundation for the germlayer theory.

 

Karl Ernst von Baer attended the University of Würzburg in 1815, where he was introduced to the new field of embryology. His anatomy professor encouraged him to pursue research on chick embryo development. Unable to pay for the eggs or hiring an attendant to watch the incubators, he turned the project over to his more-affluent friend Christian Heinrich Pander. Pander identified three distinct regions in the chick embryo, which von Baer extended in 1828 to show that in all vertebrate embryos, there are three concentric germ layers.

Candling Eggs: A Technique to Observe Embryo Development and Veins

In 1842, the Polish-German embryologist Robert Remak provided microscopic evidence for the existence of these layers and designated them by names still in use. The ectoderm or outermost layer develops into the skin and nerves, while from the endoderm, the innermost layer, comes the digestive system and lungs. Between these layers, the mesoderm is derived blood, heart, kidneys, gonads, bones, and connective tissues. It was later discovered that all vertebrates exhibit bilateral symmetry and have three germ layers. Animals that display radial symmetry, such as hydra and sea anemone, have two layers, while only the sponge has a single germ layer.