Animal Development.Living animals are constantly on the move. It is one of the most characteristic things about them. Often we can see them running about, breathing, catching food and eating it, and so on. If we look closer we find that an animal is made up of different organs, and in all of them there is something going on all the time. On an even smaller scale, the organs are built out of cells, little lumps of living matter, each containing a special kernel or nucleus. And each cell is always full of activity. In plants the living jelly streams slowly about from one side of the cell to the other: in animal cells we cannot usually see any movement, but nevertheless there are incessant chemical actions and reactions. The cell absorbs oxygen and other substances from outside, performs many complicated chemical operations with them, and pours out again into its surroundings the by¬ products for which it has no use.
In a living organism these changes are not isolated but are adjusted to one another so that the right operations are carried out to produce the right quantities of the various products. It is because we are so impressed at the way in which all the separate processes work together harmoniously that we call animals “organisms.” The processes which keep an animal alive have to be quite as highly organized as the operations in the most complicated mass-production factory. If there is a “secret of life,” it is here we must look for it, among the causes which bring about the arrangement of innumerable separate processes into a single harmonious living organism.
When a numerous and varied set of processes is to be organized it is obviously convenient, and often absolutely necessary, to separate the different jobs among different pieces of apparatus, each of which specializes in carrying out one particular function. Thus a motor-car has a separate apparatus the carburetor to vaporize the fuel, another apparatus the dynamo to provide electric power, still another the sparking plug—to make a spark, and so on. We find the same sort of plan adopted in all animals which attain more than a very minute size. For instance, every living creature has to arrange to absorb oxygen from its surroundings and to transport it in the right quantities to the cells in the body which need it. We find that there are special organs for absorbing it, lungs in animals which breathe air, gills in animals which absorb the oxygen dissolved in water; special organs, the blood-vessels, for transporting the oxygen all through the body after it has been absorbed and dissolved in the blood.
To say that an animal is an organism means in fact two things: firstly, that it is a system made up of separate parts, and secondly, that in order to describe fully how any one part works one has to refer either to the whole system or to the other parts. Thus it is impossible to describe fully a thighbone without referring to the fact that it is part of a leg, and that one end fits on to a pelvis and the other on to a shinbone. The relation with the other parts of the organism is indeed so close that if an anatomist finds a new fossil bone he can often reconstruct, in general outline, the whole unknown animal to which it belongs.
There are two possible ways of investigating the organization of an animal. Firstly, we can study in the adult how the organism works as a going concern: we can find out what functions are per¬ formed by each separate organ; we can discover how the communications between the organs are maintained by the blood and nerves; and we can study the results of removing one or more organs. But all the processes which can be investigated in this way will be proceeding within the framework provided by the fundamental spatial pattern in which the parts of the animal are arranged, since in the adult this pattern is more or less fixed.
But there is a second line of attack. We can actually watch how the parts of a living organism come into being and fit together. Nearly all organisms start life as fertilized eggs, though a few grow out as buds from other organisms. Fertilized eggs are very simple-looking, often apparently quite homo¬ geneous lumps of living matter. They consist of a watery jelly, ihc protoplasm^ which contains a variable amount of food-matter or yolk^ and which also encloses a little bag of special material which is the kernel or nucleus. As we shall see, the jelly-like protoplasm is not really as simple as it looks. But it is at any rate much simpler than the adult animal, which consists of very large numbers of cells, of several different kinds, arranged in various ways to build up the different organs. During the increase in complexity as the egg develops into the adult the spatial pattern of the animal arises. In the early stages it is fluid and unfixed; we can describe its gradual unfolding, make experiments which alter it, study its genesis and causation.
