Development is the process by which a single cell (zygote) is transformed into an adult organism. Sometimes the term is extended to include changes undergone by the adult organism itself - such as aging and death. The study of the early stages of development--those that occur before birth in mammals and before hatching in oviparous animals--is called embryology.  Embryology and development have been active fields of biological research and investigation for well over a century. They are among the older and more well-established areas of specialization in biology. If you had been a biology major in the year 1900, you could have taken a course in embryology or developmental biology, but you would have searched your college catalogue in vain for a course in cell biology, molecular biology or even genetics!

During the years of research on embryology, the development of many animals, especially vertebrates, have been studied and described in great detail. Some intriguing facts about how the process is guided and regulated have been uncovered. Nevertheless, to this day many aspects of developmental processes (in animals, plants, or fungi) remain unknown.

(A) Fertilization and Cleavage

The event that initiates the process of development is of course fertilization - the fusion of two haploid gametes to form a diploid zygote. Following fertilization in animals the zygote undergoes a number of cleavage events. These are mitotic divisions in the course of which the number of cells that make up the developing embryo increases, but the overall size of the embryo does not. This means that the size of each cell decreases progressively with each successive mitotic division. The remainder of the process of development consists of three components: growth, differentiation, and morphogenesis. These processes occur simultaneously during development, and each presents baffling problems to biologists.

(B) Growth

The simplest of the three components of the development process is growth. This involves an increase in the size of the organism, often accompanied by an increase in the number of cells in the organism. The biosynthetic processes by which a cell can increase in size are fairly well understood by now, as is cell division, the process by which cells increase in number. But exactly how are these processes turned on and off at the appropriate times? Why do tigers, for example, not just keep getting bigger and bigger all their lives, like trees do? No complete satisfactory answer to these and other similar questions has been found. In addition to their great intrinsic scientific interest, such answers might have important practical applications. For example, it might be that tumors are groups of cells in which something has gone wrong with whatever mechanisms turn cell growth on and off at the right times.

(C) Differentiation

Differentiation is the process by which cells of a developing organism assume different and highly specialized forms and functions. An adult vertebrate consists of several hundred million cells, and these cells can be very different from one another. A nerve cell is different from a muscle cell or an epithelial cell in both form and function. Nevertheless, these cells developed from a single zygote whose genetic composition was fixed at the moment of fertilization. Furthermore, that genetic composition is passed on unchanged to all cells that derive from the zygote. Therefore nerve cells, muscle cells and epithelial cells of an adult organism, despite their differences, share the same genetic blueprint. Muscle cells have genes that enable them to make myosin and actin. However, the nerve cells and epithelial cells also possess the genes for myosin and actin, only they are rarely turned on, and these cells produce very little actin or myosin. Moreover, even the muscle cells do not produce actin and myosin all the time, but only when it is needed. Only a very complex system of controls and regulatory mechanisms could explain how the right genes are turned on and off at the right times in the right cells.

(D) Morphogenesis

A complex multi-celled organism is much more than just a random aggregation of different types of cells, however. These cells stand in very definite spatial relations to one another. Your muscle cells do not lie exposed on the surface of your body, nor are your nerve cells gathered together in a clump in the middle of your abdomen. The process by which the cells of a developing organism take up their proper places or positions with respect to one another is called morphogenesis (Gr: morpho = form, gene = origin). Once again, exactly how this occurs remains unclear. It involves directed movement of cells, and also some system of communication among cells. This communication system may be chemical, involving secretion of hormone-like chemical messengers or signals by different cells at different times; or it may involve proteins and oligosaccharides found on the surface of cell membranes that somehow enable cells to "recognize" one another.