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Illustration of the animal-vegetal gradient in Xenopus laevis ( African clawed frog) eggs after fertilization. During fertilization, the sperm s point of entry determines the future dorsal side (shaded) and ventral side (unshaded) of the embryo. The prospective ventral side of the embryo forms on the side where the sperm enters while the prospective dorsal side forms opposite the sperm s point of entry.
The male body, followed by male reproductive organs from which the sperm originates, is depicted from top to bottom at the left. Under the male reproductive organs is a diagram of a single sperm. To the right of the sperm diagram, the physiological and morphological changes a sperm undergoes to fertilize an egg are depicted from left to right. Each change is associated with a light pink rectangle background. Each light pink rectangle corresponds to the location of the sperm within the female reproductive organs, which is depicted above it.
Fetal programming, or prenatal programming, is a concept that suggests certain events occurring during critical points of pregnancy may cause permanent effects on the fetus and the infant long after birth. The concept of fetal programming stemmed from the fetal origins hypothesis, also known as Barker’s hypothesis, that David Barker proposed in 1995 at the University of Southampton in Southampton, England.
The French flag model represents how embryonic cells receive and respond to genetic information and subsequently differentiate into patterns. Created by Lewis Wolpert in the late 1960s, the model uses the French tricolor flag as visual representation to explain how embryonic cells can interpret genetic code to create the same pattern even when certain pieces of the embryo are removed. Wolpert's model has provided crucial theoretical framework for investigating universal mechanisms of pattern formation during development.
Among other functions, the Notch signaling pathway contributes to the development of somites in animals. It involves a cell signaling mechanism with a wide range of functions, including cellular differentiation, and the formation of the embryonic structures (embryogenesis). All multicellular animals use Notch signaling, which is involved in the development, maintenance, and regeneration of a range of tissues. The Notch signaling pathways spans two cells, and consists of receptor proteins, which cross one cell's membrane and interacts with proteins on adjacent cells, called ligands.
Between 1934 and 1945, George Beadle developed a hypothesis that each gene within the chromosomes of organisms each produced one enzyme. Enzymes are types of proteins that can catalyze reactions inside cells, and the figure shows that each enzyme controls a stage in a series of biochemical reactions. The top box in this figure represents a normal process of enzyme production and biochemical reactions, and the bottom box shows how Beadle's experiments affected the normal biochemical process.