408 



A re-examination of my material of former years has shown that 

 in several cases at least I had obtained this same result and had 

 described the new part as an anterior end with many new segments. 

 Rievel and Korschelt have also both given figures of pieces of worms 

 having a large number of anterior segments. It is somewhat hazardous 

 interpreting the results of others based on their figures, yet so similar 

 are these figures of Rievel and of Korschelt to the posterior ends 

 that I have examined that it seems highly probable that Joest, Rievel, 

 and Korschelt have also mistaken for a head end what is in reality 

 a tail. If this interpretation is correct then the figures given by two 

 of these authors represent posterior and not head ends, and conse- 

 quently many of the objections that they have raised against the re- 

 sults of Hescheler and myself fall to the ground. Sections of the 

 worms figured by Rievel and by Korschelt will at once reveal 

 whether anterior or posterior ends are present. 



This discovery of Spallanzani (1768) is the earliest record, so 

 far as I know, of the replacement of a part of the body by a part 

 different in kind. Loeb has described (1891) several such cases in 

 hydroids and proposed the name heteromorphosis for the process. 



The evidence on which I have based the statements above is 

 drawn from the following facts. 



In an experiment begun on Nov. 9 ('97) and brought to a close 

 on May 13 ('98) a number of worms were cut in two about 30 seg- 

 ments from the anterior end i. e. just behind the girdle. Several of 

 the posterior parts of these worms have regenerated (at the anterior 

 end) a new part formed of a number of small segments. One of these 

 pieces is drawn in Figs. 1—4. Figure 1 shows the entire piece. It 



Fig. 2. Fig. 3. Fig. 4. 



is formed of 64 old segments and 21 new segments — the new part 

 measures about 3| mm in length. Figure 2 shows the anterior end, 

 with the new part, drawn to a larger scale (X 4). Figure 3 shows 

 the tip of the new part seen from the end ; and Figure 4 is the same 

 tip seen from above. The opening at the end is slit like and longest 

 from above downwards. It resembles the anus of the normal worm. 



