ABSTRACT
We will commence our survey with the concept of individuality. The literal meaning of “individual” is that which is indivisible. In the case of man and the higher animals indivisibility is largely a fact. It may first be said that we experience ourselves subjectively at every moment as an indivisible unity. Certain parts of the body, for example, arms or legs, can be removed from man or higher animals, without his life being directly endangered thereby; but the part cut off perishes. Such an operation, therefore, does not give us two parts each capable of separate life. And if we were to attempt to cut one of the higher organisms into two parts of approximately equal size, we should be met with complete failure, inasmuch as both halves would then immediately perish. However, by suitable treatment it is possible to maintain alive for a considerable time parts separated from the bodies of the higher and even the highest animals. The isolated heart, for example, of a frog or other vertebrate continues to beat if we supply it with a suitable salt solution in place of blood. Success has also been obtained in removing parts of the bodies of higher vertebrates—for example chicken embryos, and cultivating them further in artificial media. In this way, it is possible to maintain for a long period cultures of connective tissue, muscle-cells, epithelia, gland cells, and so on, and even to cause them to multiply greatly. Nevertheless, the characteristic structure of the organs in question, which would otherwise be built up by these cells, is lost, and the cells themselves may change their nature very greatly. They grow into nothing more than an irregular mass of cells. All the same, we are obliged to conclude that even the highest organism is not indivisible in the strictest sense, since it is possible to grow single parts of its structure in a suitable culture medium; this method is called “explantation”.
