Obviously enough, the concentration of these substances will be high close to the cells that are making them, and lower as they diffuse further away. These special substances are called morphogens, because they influence the shapes of the structures that the cells of the embryo will form. To the developmental biologist, the appeal of this idea lies in its simplicity but, as is often the case, it is the simplest ideas which are the hardest to prove. Thus, although there have been tantalising hints over the last twenty years that morphogens do exist, there has been no definitive proof. It is only in the last eighteen months that good evidence has been obtained.
Developmental biologists work on frog embryos for many reasons.
The early stages of frog development are like those of any other species, including man. After fertilisation, when the sperm meets the egg, the egg begins to divide. At this time, the embryo is just a ball of cells, with no obvious separation into different tissues. It turns out, however, that already the embryo has formed some different types of cell that can be recognised as different only because they make special proteins. These cells do not form the sorts of tissues we are familiar with, like muscle, bone, or blood.