Of course there is another cause of disintegration, namely, microörganisms from the air and in the intestines. During life the cells are protected by a normal membrane that is destroyed in death and then the action of the microörganisms can superpose itself upon that of digestion. Thus in man death is stopping the breath and this may be done by poison, disease, etc. The problem is whether there is any natural death, for if not we ought to be able to prolong life indefinitely. But we cannot experiment on man because neither the intestines nor respiratory tract can be kept free from microbes. A Russian, Bogdanow, solved this problem for the fly, putting its fresh eggs into bichlorid of mercury, which a few survived, with no microörganisms on the outside. These eggs were then developed on sterilized meat in sterile flasks and Guyemot raised 80 generations of fruit flies thus. Loeb himself and Northrop raised 87 generations. Their dead bodies were transferred to culture media such as are used for the growth of bacteria and more were produced thus for years. Hence fruit flies freed from infection and well fed would not entirely escape death and probably higher organisms would thus die from internal causes were external ones excluded. Eggs, for example, those of starfish, ripen and disintegrate very rapidly if not fertilized by the process of autolysis, which acts only after the egg is ripe. The fertilized egg, however, does not degenerate in the presence of oxygen but dies in its absence, so that we might say that the fertilized egg is a strict aërobe and the unfertilized, an anaërobe. The entrance of the spermatozoön saves the life of the egg.