It is scarcely necessary to say that the ciliate Infusoria can propagate without previous coition. The agamic mode of reproduction appears to be almost the same, save in a few details, as that which follows coition. It consists in a bipartition or division of the body of the animal along a plane usually perpendicular to the grand axis of the nucleus, and it is a matter of course that that element takes part in the division at the same time with the protoplasm. These phenomena of reproduction it is possible to study upon a grand scale by supplying Infusoria kept in captivity with abundance of nourishment. The easiest way is to produce a putrid fermentation by means of vegetable fragments crushed and macerated in water. The Infusoria contained in this water find abundant food furnished by the bacteria developed in it, and they therefore multiply in great numbers. By means of appropriate methods of treatment and isolation we are able to follow the phenomenon step by step and to examine what the animal actually becomes after each agamic bipartition. Weismann, when he laid the foundation of his theory of the immortality of Infusoria, supposed that the development of the Infusoria by bipartition had no limits and could be prolonged indefinitely without injury to the vitality of the protoplasm. Various authors had already made observations which were directly in contradiction with this view. M. Balbiani, in 1860, in a communication entitled, "Observations and Experiments upon the Phenomena of Fissiparous Reproduction among Ciliate Infusoria,"[13] concludes thus: "one of the most important questions ... has been to determine whether this mode of propagation is really unlimited, or whether, after being continued throughout a greater or lesser number of generations, it becomes by degrees enfeebled, finally to disappear completely.... We have established that this mode of propagation has its limits, and ends invariably in one of the three following ways: either by the _natural_ and almost simultaneous _death_ of all the individuals belonging to the same cycle, or by the recurrence of sexual generation leading to the termination of one of the cycles and the commencement of a new cycle, or finally by the phenomenon of encystment, which in fact brings about only a momentary interruption of the process of reproduction by fissiparity" M. Balbiani, apropos of this subject, has called attention to a curious observation made by the celebrated Danish micrographer O. F. Müller, who lived in the last century. Müller had observed that the individuals of any one species most ordinarily found in coition were almost all of small stature. But he took them for the young individuals of the species. Now these individuals of small size are in reality the oldest, that is to say, they are the ones that are the result of a great number of successive bipartitions; and it is to be observed, that, in a great many species, in proportion as the bipartitions