Embryology is the study of how organisms develop from a single cell to a multicellular form. The term embryology is often used interchangeably with the term developmental biology; embryology, however, connotes a more classical approach to the field and is restricted to the study of the stages of development from fertilization of the egg to hatching, or birth. By contrast, developmental biology connotes a more modern approach and encompasses the entire life cycle of an organism, including the differentiation of egg and sperm, reproduction, embryogenesis, hatching or birth, metamorphosis, aging, and death. In spite of the different names, however, both embryology and developmental biology share a single common goal of unraveling the complex biological manner in which a multicellular organism is constructed.

Embryology can be divided into three major subfields. The first and oldest subfield of embryology is descriptive embryology. A descriptive embryologist is interested in understanding the basic structural patterns of the embryo. This can be accomplished by simply observing a developing embryo, using either a microscope or the naked eye. Often, however, the embryo will be preserved at different stages; it is analyzed by preparing thin cross-sections, mounting the sections onto glass slides, and observ ing the sections through a microscope. Modern approaches to descriptive embryology include using time-lapse cinematography or advanced microscopy to observe developing embryos.

Another subfield of embryology is experimental embryology. An experimental embryologist is interested in understanding how development works by posing hypotheses and testing them through the controlled manipulation of embryos. Experiments typically involve removing tissue from a developing embryo, grafting that tissue to another part of the embryo, and observing the effects. The study of developmental regulatory genes— genes that control developmental processes— also falls under the umbrella of experimental embryology. A genetic approach to experimental embryology includes locating or manipulating a particular developmental regulatory gene and observing its effects on the developing embryo.

The third subfield, comparative embryology, involves the study of two or more different species of organisms in order to gain insight into the similarities and differences in their development. Comparative embryolo-gists are typically interested in investigating the embryo's role in evolution. The basic assumption behind comparative embryology is that new life forms must have evolved through changes in the developmental program of their ancestors, and insight into evolution may therefore be gleaned by comparing the development of diverse organisms. Understanding where and when the developmental programs diverge between distinct organisms has been a traditional approach to the subfield. More recently, understanding the role of developmental regulatory genes in diverse organisms has been the focus of comparative embryological studies (see Embryology Today, below).

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