An unusually favourable object for the study of natural death is the animal egg. The egg of the starfish _Asterias forbesii_ when taken out of the body is usually immature, but in the spawning season it ripens in sea water. The writer[306] observed that eggs which ripen disintegrate very rapidly when not fertilized. This disintegration may be due to a process of autolysis, which sets in only after the egg has extruded the two polar bodies. The writer found that by preventing the maturation of the egg either by withdrawing the oxygen or by replacing the alkaline sea water by a neutral solution or by exposing the eggs for some time to acidulated sea water, the disintegration could also be prevented. [306] Loeb, J., _Biol. Bull._, 1902, iii., 295. Further experiments showed that even in the mature egg rapid disintegration could be prevented by lack of oxygen, and similar results were obtained by Mathews. When the egg is fertilized it does not disintegrate in the presence of oxygen but it gradually dies in the absence of oxygen. One is almost tempted to say that while the fertilized egg is a strict aërobe the mature unfertilized egg is an anaërobe. This latter statement, however, becomes doubtful since the presence of oxygen may help the disintegration only indirectly by allowing certain changes to go on in the egg. The important points for us are that duration of life in the mature unfertilized egg is comparatively short and that the entrance of a spermatozoön or the process of artificial parthenogenesis saves the life of the egg. Loeb and Lewis found that the life of the unfertilized sea-urchin egg (which is usually mature when removed from the ovaries) can also be prolonged when its oxidations are suppressed. The decay of the unfertilized egg seems to be due to the fact that those alterations in the cortical layer which underlie the membrane formation and which are responsible for the starting of development gradually take place. In such a condition the egg will die quickly unless deprived of oxygen. This view is supported by the observation of Wasteneys that unfertilized eggs of _Arbacia_ show an increased rate of oxidations when allowed to remain for some time in sea water; we have seen in Chapter V that such an increase also accompanies artificial membrane formation. 6. If the limited duration of life of an organism is determined by one or more definite harmful chemical processes, we should expect to find a temperature coefficient for the duration of life or at least be able to show that if all other conditions are the same the duration of life is for a given organism a function of temperature. The writer[307] investigated the duration of life of fertilized and unfertilized eggs of _Strongylocentrotus purpuratus_ for the upper temperature limits. [307] Loeb, J., _Arch. f. d. ges. Physiol._, 1908, cxxiv., 411. TABLE XX