KEPRODUCTION AND HEREDITY 255 



the rudiments (Anlagen) of the organs of the great-grand- 

 children. 



This division with preceding re-growth of the organs occurs, 

 in addition to the Turbellarians, among quite a large number of 

 the individuals of other classes ; for instance, various tunicates, 

 numerous jelly-fishes and sponges reproduce in a similar manner, 

 when fission may take place either transversely or longitudinally. 

 But as in these organisms division is frequently incomplete the 

 result is the formation of larger and smaller stocks. Sponges, 

 in particular, are as a rule composed of a large number of 

 individuals whose relations are very frequently so close that they 

 appear to us as one individual. A division followed by regenera- 

 tion is also observed in several worms and also many starfishes. 



Very remarkable are those cases in which the young do not 

 await their full development, but proceed to sexual reproduction 

 even at an early embryonic stage. Kleinberg states that the 

 embryos of Lumbricus trapezoides, a relative of our earthworm, 

 divide shortly after the gastrula stage. In this manner two 

 individuals regularly originate from each ovum. In exceptional 

 cases a longitudinal division will take place in embryos of verte- 

 brates. This usually gives rise to strange duplicate malforma- 

 tions, but leads sometimes to the formation of normal, healthy 

 twins. 



These conditions are most striking in the case of Encystus, 

 one of the Ichneurnonidse. This little robber searches usually for 

 butterfly-eggs into which it lays one egg by means of its ovipositor. 

 Before this proceeds to develop it first divides into a considerable 

 number of cells from each of which, if conditions be favourable, 

 there arises a larva and later a young ichneumonid. 



While during reproduction by fission the mother-individual 

 is completely divided into the two young, and therefore ceases to 

 exist as an independent individual, it continues in the budding 

 process to exist as an independent individual even after the 

 separation of the bud. Generally speaking, fission generates 

 equivalent products, budding non-equivalent products. But this 

 must not be taken too literally, for as Nature is always an enemy 

 of sharp contrasts so a gradual transition exists between these 

 two forms of asexual reproduction, and in numerous cases it is 

 left to our choice to describe the process either as budding or as 

 fission. 



