EMBRYO DEVELOPMENT
The field which deals with the development of multicelled organisms is called developmental biology. F.H.C. Crick, the co-discoverer of DNA replication, has defined the field as follows :
This short description gives some idea of the enormous complexity of the process of embryonic development. Each embryo starts out as a single fertilized egg cell. As the cells divide and divide again, they reach a point where different types of cells :bone, blood, muscle, tissue, etc., are required. Somehow the cells seem to know how and when to form the required types, and also how to arrange themselves into the required structures. It is believed that the genes in each cell control this process, and that each cell is able to sense its position in the embryo and turn itself into the correct type of cell at the proper time. Sir Vincent Wigglesworth has described the role of the genes as follows :
Experiments have proven that each cell acts according to its own internally stored set of instructions. When cells from a developing animal embryo are experimentally moved to a different location during early stages, they are able to transform themselves into the appropriate size, shape and type to fit in with their new location. Similarly, certain animals are able to regrow severed limbs from the cells at the stump. And finally, a recent experiment successfully cloned an entire new toad by using the nucleus of a cell from the lining of the intestine to replace the nucleus of an egg cell.
These cases suggest several important things about the nature of the gene instructions inside each cell. These instructions stored in the genes are the same in each cell of the body, and are the complete set, capable of regrowing an entire individual. Only portions of this complete set of instructions are normally used by any particular cell, but each has access to the complete set if it is relocated as an embryo or later needs to regrow a severed limb. Information is passed to the genes from the world outside, so that they can tell the cell when to divide or to transform into a new type.
From the article, "Computers and Evolution", 1983.