Asexual or monogenetic tocogony (also called "vegetative multiplication") is always effected by a single organic individual, and so must be traced to its transgressive growth. When this affects the entire body as a total growth, the whole dividing into two or more equal parts, we call the monogenetic process division (or segmentation). But when the growth is partial, and affects only a part of the individual, or when this special part separates from the generating organism in the form of a bud (_gemma_), the process is called budding or gemmation (_gemmatio_). Hence the essential difference between the two forms of generation is that in division the parent disappears in its partial products (children); these are of the same age and form. But in budding the generating parent retains its individuality; it is larger and older than the young bud. This important difference between division and gemmation, which is often overlooked, holds good both for protists (unicellulars) and histona (multicellulars). The fact that in division the individual as such is destroyed is a sufficient refutation of Weismann's theory of the immortality of the unicellulars. (See above, and also the _Riddle_, chapter xi.) Reproduction by division is by far the most common of all forms of propagation. It is the normal form of monogony, not only in many of the protists, but also in the tissue-cells which compose the tissues of the histona. It is, moreover, the sole method of propagation for most of the monera, both chromacea and bacteria, which are in consequence often comprised under the title of "cleavage-plants" (_schizophyta_). Self-cleavage is also found among the higher multicellular organisms--namely, the cnidaria (polyps, medusæ). It usually takes the form of division into two parts (_dimidiatio_ or hemitomy), the body splitting into two equal halves. The plane of division is sometimes indefinite (fragmentary-cleavage), sometimes coincident with the long axis (length-cleavage), sometimes with the transverse axis, vertical to the long axis (transverse-cleavage), and less frequently with a diagonal axis (oblique-cleavage). When the segmentation of a cell proceeds so rapidly that the transverse-cleavage follows immediately on the length-cleavage, and the two are at length made to coincide, twofold division is changed into fourfold division. And when the process is repeated in quick succession, and the body falls at last into a number of small and equal pieces, we have manifold division (polytomy); as in the spore-formation of the sporozoa and rhizopoda, and in the embryonic sac of the phanerogams.