144 RE GENERA TION 



more segments at its posterior end. Reproductive organs appear in 

 each individual, and when the germ-cells are mature the chain 

 breaks up. 



None of the earthworms propagate by self-division, although occa- 

 sionally, under unfavorable conditions, pieces may pinch off at the 

 posterior end. 1 Lumbriculus, on the other hand, propagates by self- 

 division, although it has been disputed whether the division takes 

 place without the intervention of an external injury or disturbance of 

 some sort, or whether the division may take place entirely from inter- 

 nal causes, that is, spontaneously. Von Wagner has shown that at 

 certain seasons lumbriculus breaks up much more readily than at 

 other times, which may only mean that it is more sensitive to stimuli 

 at one time than at another. 



The pieces into which lumbriculus breaks up regenerate after 

 separation. In another form, Ctenodrilus monostylos, division takes 

 place first in the middle of the body behind a cross-septum. Each 

 half may again divide in the same way, and the same process may be 

 repeated again and again until some of the pieces are reduced to a 

 single segment. A new anterior and posterior end may then develop 

 on each piece. In Ctenodrilus pardalis each segment of the middle 

 region of the body constricts from the one in front and from the one 

 behind, and each produces a new head at its anterior end and an anal 

 opening at its posterior end. The worm then breaks up into a num- 

 ber of separate worms. In this series, self-division of the individual 

 is not associated with the development of sexual forms, but seems to 

 be a purely non-sexual method of reproduction. In the leeches self- 

 division does not occur, and no cases are known in the mollusks. 



In the echinoderms several forms reproduce by voluntary self-di- 

 vision. In the brittle-stars some forms divide by the disk separating 

 into two parts, one having two and the other three of the old arms. 

 Each piece of the disk then regenerates the missing part of the disk 

 as well as the additional arms. In the starfishes the arms may be 

 thrown off if injured, and, while in certain forms the lost arm does 

 not regenerate a new disk, yet, according to several writers, it 

 may in other species regenerate a new animal. Dalyell observed a 

 process of self-division in a holothurian, each part producing a new 

 individual, and more recent observers have confirmed this discovery. 



No cases of self-division are known in the groups of myriapods, 

 insects, crustaceans, spiders, polyzoans, brachiopods, enteropneusta, 

 or vertebrates. 



Before discussing the general problems connected with the pre- 

 ceding cases, I should like to point out that it is certainly a striking 

 fact that in all, or nearly all, of these cases of self-division, the sepa- 



1 See Hescheler ('97). 



