452 LIFE : OUTLINES OF GENERAL BIOLOGY 



This process seems to maintain vigour and to promote variability 

 in the stock. Here experiments have been of interest. In artificially 

 isolated "pure lines" (all descended from one), no conjugation 

 occurs, yet there is a periodic (it may be monthly) occurrence, in 

 many or most individuals, of a remarkable process (endomixis), 

 in which the nuclear organisation is disintegrated and reconstructed. 

 The individuals behave as if they were about to conjugate with one 

 another, but no conjugation occurs. This disintegration and rein- 

 tegration here suffice to secure continuance of vigour — in fact 

 rejuvenescence. 



Among many-celled organisms, both plants and lower animals, 

 there is frequently a completely asexual reproduction, in which an 

 existing part of the parent or a new growth from it is separated 

 off to start a new individual. Thus the freshwater Hydra forms 

 buds, which attain adult form before separation; a sea-anemone 

 may not only bud, but may divide vertically into two; a ribbon- 

 worm (Nemertean) may break into half a dozen pieces; two or 

 three kinds of starfish can normall}' multiply by separating off their 

 arms ; a liverwort may give off multicellular gemmae and a tiger-lily 

 drops its bulbils. In short, there is great variety of asexual multi- 

 plication; and this often leads to the formation of colonies, as in 

 zoophytes and corals, Bryozoa, and compound Tunicates. 



But all the many-celled animals and the great majority of many- 

 celled plants show sexual reproduction; though the asexual mode 

 also often occurs in the same organism. Thus while the main mode 

 of multiplication in the freshwater Hydra is the liberation of 

 asexually-produced buds, there is also sexual reproduction by eggs 

 and sperms. Here we have to note: (a) the formation and segre- 

 gation of special reproductive cells, (b) the production of the two 

 different kinds of special reproductive cells, and (c) the production 

 of these (the ova and spermatozoa) by different (male and female) 

 organs or individuals. In plants like ferns and mosses, one phase 

 in the life-history is the formation of spores — reproductive cells 

 arising apart from gonads and capable of developing without fertili- 

 sation. Such cells occur among animals also, as in the life-history 

 of the hver-fluke, where the larvae multiply by spore-cells, while 

 the adult reproduces as usual by ova and spermatozoa (see Alter- 

 nation of (ienerations). 



In some animals, such as certain Rotifers, males are not 

 known; and in some other types they are absent for long periods 

 (as in summer greenflies), or seeming unnecessary even when they 

 are present. In other words. Parthenogenesis often occurs; yet 

 this may best be thought of as the unisexual mode of sexual 

 reproduction. For although there is no fertilisation, there is multi- 

 plication by means of egg-cells. Many common animals, such as 

 snails, earthworms, and leeches, are hermaphrodite; that is to say, 



