64 REGENERATION 



at the anterior end of the piece, and sometimes also at the basal end. 

 A similar statement may be made for each of the other kinds. 

 Another method of regeneration sometimes takes place, when, for 

 instance, a piece of the stalk of a nutritive individual is left undis- 

 turbed without being supplied with fresh water. It sends out root- 

 like stolons instead of producing a new zooid. The stolons appear 

 first at the ends of the piece, but may later also appear at several 

 points along the piece. They make a delicate network, and the origi- 

 nal piece may entirely disappear in the stolons. After several days 

 new feeding zooids grow out at right angles to the stolon network. 

 Pieces of the stalk of protective zooids may also produce stolons, but 

 they spread less slowly, and the formation of new individuals was not 

 observed. In one case a piece of a reproductive zooid made a short 

 stolon, and from it arose a new individual that seemed to be a nutri- 

 tive zooid. If the latter result proves to be true, we see that a piece 

 may produce a new part that is of a different kind from that of which 

 the piece itself was once a part, but this is brought about by the forma- 

 tion of a stolon that is itself one of the characteristic structures by 

 means of which these colonial forms produce new nutritive zooids. 

 In this case there is a return of the piece to a simpler form, the stolon, 

 and, acting on this, the factors that produce nutritive zooids may bring 

 about new nutritive zooids. The influence of the old structure is lost 

 when the piece assumes a new character. 



Another series of experiments gives an insight into an internal 

 factor of regeneration that may prove, I think, to be one of some 

 importance and help in interpreting certain phenomena. If the 

 head-end of a planarian is cut off, the posterior piece split along 

 the middle line, and one side cut off, just above the lower end of the 

 longitudinal cut, as shown in Fig. 31, A, it will be found that, if the 

 long and the short sides are kept from uniting along the middle 

 line, each half will produce a new head on its anterior surface (Fig. 

 31, C). If the two halves grow together, and the anterior surface of 

 the shorter piece becomes connected with the anterior surface of the 

 longer piece by means of the new tissue that develops along the 

 inner side of the latter (Fig. 30, B\ then a head appears only 

 on the anterior half. The development of a head on the shorter 

 half is prevented by the establishment of a connection with the 

 new side. Sometimes an abortive attempt to produce a head is 

 made, but the posterior surface fails to produce anything more than 

 a pointed outgrowth. If we attempt to picture to ourselves how this 

 influence of the new side on the posterior surface is brought 

 about, we can, I think, most easily conceive the influence to be due 

 to some kind of tension or pull of the new material which is of such a 

 sort that it restrains the development of a head at a more posterior 



