2/0 C. M. Child. 



of the right head and the longitudinal tension exerted upon it are 

 the mosi important factors for the formation of the tail, as was 

 shown to be the case in Stenostoma (Child, '03a). It is difficult 

 otherwise to understand why a bilaterally symmetrical posterior 

 end should arise from the extreme lateral portion of the body. 

 The cases of formation of supernumerary posterior ends in 

 Planaria and other forms are without doubt similar in character. 

 An account of certain experiments along this line will be given at 

 another time. 



2. Experimental Duplication of the Posterior End. 



Attempts at duplication of the posterior end by partial longitudi- 

 nal splitting succeed only rarely because the use of the margins 

 and posterior ends for attachment during locomotion is such as to 

 press the tw^o cut surfaces closely together and union almost 

 invariably occurs within a few days, no matter how often the 

 operation is repeated. A few cases showing some degree of dupli- 

 cation of the posterior end were obtained, but one case w^as of 

 special interest since it indicates the importance of the mechanical 

 tension as a factor in the regeneration of the posterior end. 



This case was one of a series of eight specimens each of w^hich 

 had been cut transversely through the middle of the body and then 

 the anterior piece split longitudinally nearly to the ganglia (Fig. 

 27). After the first operation all the pieces united again, but after 

 the second operation one piece was found in which the union was 

 not complete (Fig. 28). In this piece the contraction of the longi- 

 tudinal cut surfaces was so great that each half of the transverse 

 cut surface which originally formed the posterior end of the piece 

 had been drawn in into a position facing the median plane, /. e., 

 at right angles to its original position. In consequence of this 

 contraction a part of the lateral margin of each half of the speci- 

 men formed the actual posterior end. The posterior portion of 

 the specimens with the two parts separated as widely as possible 

 is shown in Fig. 29 on a larger scale ( X 14, the other figures X 7). 

 Here it is seen that the longitudinal cut surfaces have united 

 except for a short distance at the posterior end. The originally 

 transverse cut surfaces, though now nearly longitudinal, can be 

 distinguished from the original longitudinal cut surfaces by their 

 concavity and by the amount of regeneration which has taken 



