Amongst Protozoa, as in the higher animals and plants, reduc tion divisions of chromosomes have also been shown to occur; in some the division takes place just before gamete-formation; in others (Volvocineae, Coccidia), shortly after fertilization. In view of what we already know of nuclear division in the Protozoa it is not at all surprising that the course of reduction division turns out to be in some cases (e.g., in Actinophrys) quite as com plex as in the higher animals. (See CYTOLOGY.) Parthenogenesis.—The egg cells of some animals and plants are known to be capable of developing without fertilization into new individuals; this kind of development we call partheno genesis (q.v.), and it is not unknown among the Protozoa. In Actinophrys for example both gametes instead of fusing can encyst separately, shutting themselves up in a capsule, and later reappear as vegetative individuals.
We have already had occasion to show that zygotes may become enclosed in a rigid membrane. But the vegetative individuals of many Protozoa are also capable of doing so. This process, called encystment, is only met with as a rule when a shortage of food sets in or when the water in which the creatures live begins to dry up. This kind of cyst we call, in contradistinction to the "fertilization cyst" referred to earlier, a "protective cyst." Apart from these we ought to mention the "reproductive cyst"; some protozoans do not divide in the free motile state, but first encyst, the products slip ping out again after division.
When about to encyst the protozoan rounds itself off, secretes a membrane—which can afterwards be strengthened—and throws off or absorbs most of its organoids, such as flagella, static fibrillae, etc. (those of course remain which can only be repro duced by division, such as the centrosome and chromatophores). Finally it exudes a portion of the sap present in the protoplasm. Encystment consists then not only in the formation of a capsule but of a process of dedifferentiation of the individual, a reversion to a quasi-embryonic condition. The membranes are to ensure that the cyst shall be able to withstand complete drying out and the action of chemical compounds such as weak acids that would kill the vegetative individual. The thickening (dehydration) of the protoplasm ensures the resistance of some protozoans to tem peratures as high as 6o° C (18o° F) which certainly could not be borne by the same species in the vegetative condition.
The encysted protozoan is in a condition of dormancy ; it has no metabolic activity that can be proved, and it can in consequence survive in this "latent" condition for a very long time.
Germination (encystment) calls for little comment. The cysts of many protozoans have weak places left in the coat at which they open as at a hinge or suture or even pores filled with plugs of silica ; but in most cases the cyst envelope bursts open like the shell of a lizard's egg (fig. II). This explosion is often due to the absorption of water by the proto plasm and its consequent swelling in the early stages of growth, but sometimes it is due to the swelling of the inner layers of the cyst membrane.
The protozoan shortly before it emerges usually reconstitutes many of its typical organoids; on swimming out it completes this process and resumes its vegetative life.
A process equivalent to the development we find in the higher animals and plants from embryo to adult is found to some extent in nearly all Protozoa with the exception of those in which the dividing mother-animal splits every one of its organoids and distributes them to its daughter-cells. But this process of de velopment, being limited in many Protozoa to the regeneration of the organoids after encystment or division (fig. 8) takes very little time and does not give any impression of complexity. Com plications that strike us as being like the early steps of embryonic development in multicellular organisms are found in the Volvocineae, at whose reproduction one cell gives rise to a colony (fig. 2, 19) . The development of other protozoans such as the foraminiferans is a lengthy process (fig. 13, 14), and in some we even find that peculiar roundabout way of de veloping, by repetition of ancestral forms, that is so often found in the higher ani mals. These processes are comparable to the formation of the gill-arches in early life by mammals (see EMBRYOLOGY and VERTEBRATE EMBRYOLOGY).
A process physiologically related to de velopment is regeneration (q.v.), the re placement of lost body parts. Some proto zoans are not capable of this ; the capacity of the others varies in degree. Some can only reconstitute their flagella, which they may have been caused by certain stimuli to cast off. Others may be cut into small pieces, and every one of these will grow up into a new indi vidual—provided of course that it has at least part of a nucleus (fig• 3)•