THE WONDERS OP LIFE 



mentation). But when the growth is partial, and af- 

 fects 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 (uni- 

 cellulars) 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, medusae). It usually takes 

 the form of division into two parts {dimidiatio or hemit- 

 omy), 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 



242 



