## Segment identity in flies

About one hundred years after Goethe wrote his treatise on plants, the zoologist William Bateson described a comparable set of abnormalities in the animal world. Bateson was convinced that the only way to understand evolution was by studying how biological forms vary, and he therefore set about cataloguing the principle types of variation in his book Materials for the Study of Variation of 1894 (Bateson was later to become one of the founders of the science of genetics, a term he coined). Among...

## Leftright asymmetry in plants and animals

There are many examples of left-right asymmetry in the living world. In some cases, the Land the R-forms occur in equal numbers, and it is largely a matter of chance whether any individual will develop as one form or the other (i.e. there is no overall handedness). Male fiddler crabs, for example, often have an enlarged claw on one side (Fig. 14.10). Whether this is on the left (L-form) or right (R-form) depends on the previous chance experiences of the animal. Young males start off with two...

## Symmetry and function

In practice, the symmetry class of an object is often related to the way it is used. Spherical symmetry, for example, predominates in objects that are used with little or no constraint from any specific direction, as with soccer balls that roll around in any way we choose. Radial symmetry, on the other hand, is common in cases where constraints operate along one major axis. Bottles and stools, for instance, are designed to withstand the effects of gravity bottles prevent liquids being spilt...

## A pattern of hidden colours

You have been invited to a football stadium with lots of other people to send a birthday message to someone flying overhead in a helicopter. The stadium has already been carefully marked out with a grid of squares, each square being painted a distinct colour, making the stadium look like a colourful patchwork. You are given two pages of instructions and a large piece of card, black on one side and white on the reverse, and told to go and stand on a square. After choosing a square, you look at...

## Proteins as guiding shapes

The properties of every plant or animal cell depend on the types of protein it contains. We normally come across proteins as part of our diet. Proteins, though, play a much more pervasive role in our lives than this might lead us to believe. All the processes in the body, such as digestion, secretion, moving, sensing and thinking, depend on the activity of different types of protein molecule. Without proteins we would not be able to do anything. The most important feature of protein molecules...

## Multicellular organisms

Fascinating though the reproduction of microbes might be, it seems to be a far cry from the way you or I reproduce. We cannot go forth and multiply by splitting ourselves in half. Our style of reproduction is more complex than that of a unicellular microbe, but nevertheless there is a common element that links the two. Both microbes and ourselves are based on the same type of building blocks, cells, but whereas individual microbes may comprise a single cell, we are made of many billions of...

## Reproduction through development

We can now return to the problem of how multicellular organisms, such as humans, mice and oak trees, reproduce. Like unicellular organisms, their reproduction is based on cell division but instead of making many separate individuals, the cells stay together to gradually build up a multicellular individual. We can summarise the whole process with a life cycle. In the case of humans, for example, parents each contribute a cell a sperm cell from the father and an egg cell from the mother. These...