The interest of the inquiry was soon directed towards testing experimentally the asserted immortality of living substance in unicellular beings. An American, named Woodruff, instituted a culture of a ciliated infusorium, a ‘slipper-animalcule’, which reproduces itself by division into two individuals; each time he isolated one of the products and put it into fresh water. He traced the propagation to the 3029th generation, when he discontinued the experiment. The last descendant of the first slipper-animalcule was just as lively as its original ancestor, without any sign of age or degeneration: if such numbers are convincing, the immortality of protozoa seemed thus experimentally demonstrable.[27] Other investigators have arrived at other results. Maupas, Calkins, etc., found, in contradiction to Woodruff, that even these infusoria after a certain number of divisions become weaker, decrease in size, lose a portion of their organisation, and finally die if they do not encounter certain invigorating influences. According to this, protozoa die after a phase of senile decay just like higher animals, in direct contravention of what is maintained by Weismann, who recognises in death a late acquisition of living organisms. Taking the net result of these researches together, we note two facts which seem to afford us a firm foothold. First: if the animalculae, at a time when they as yet show no signs of age, have the opportunity of mingling with each other, of ‘conjugating’—afterwards again separating—then they remain exempt from age, they have been ‘rejuvenated’. This conjugation is doubtless the prototype of sexual propagation of higher organisms: as yet it has nothing to do with multiplication, it is confined to the mingling of the substances of both individuals (Weismann’s Amphimixis). The invigorating influence of conjugation can also be replaced, however, by certain modes of stimulation, changes in the composition of the nutrient fluid, raising of temperature, or shaking. The famous experiment of J. Loeb will be recalled, who by the application of certain chemical stimuli to the ova of sea-urchins brought about processes of division which usually take place only after fertilisation.