[67]. Among the Rhizopoda encystment is only known in fresh-water forms, and not in a single one of the far more numerous marine forms which possess shells (see Bütschli, ‘Protozoa,’ p. 148); the marine Rhizopoda are not exposed to the effects of desiccation or frost, and thus the strongest motives for the process of encystment do not exist, at least among forms possessing a shell.
[68]. I trust that it will not be objected that the germ-cells cannot be immortal, because they frequently perish in large numbers, as a result of the natural death of the individual. There are certain definite conditions under which alone a germ-cell can render its potential immortality actual, and these conditions are for the most part fulfilled with difficulty (fertilization, etc.). It follows from this fact that the germ-cells must always be produced in numbers which reach some very high multiple of the necessary number of offspring, if these latter are to be ensured for the species. If in the natural death of the individual the germ-cells must also die, the natural death of the soma becomes a cause of accidental death to the germ-cells.
[69]. l. c., p. 78.
[70]. l. c., p. 47.
[71]. ‘Entwicklungsgeschichte der Unke,’ Leipzig, 1875, p. 65.
[72]. Id., p. 842.
[73]. ‘Ursprung des Todes,’ p. 79.
[74]. l. c., p. 42.
[75]. ‘Contributions à l’histoire des Mesozoaires. Recherches sur l’organisation et le développement embryonnaire des Orthonectides,’ Arch. de Biologie, vol. iii. 1882.
[76]. l. c., p. 37.