ZooL— Vol. II.] RITTER-CONGDON—STENOSTOMA. 367 



4. Nomenclature Employed. 



To facilitate reference to the various elements involved, 

 we use the following abbreviations : 



F. P. — normal fission plane. 



A. Y. — anterior young worm, 



P. T. — posterior young worm. 



a. c. — anterior cut. 



■p. c. — posterior cut. 



a. f. p. — anterior fission piece. 



p. /• -p. — posterior fission piece. 



a. p. — anterior piece. 



-p. p. — posterior piece. 



All normal elements, it will be noted, are indicated by 

 capital letters and all artificial ones by small letters (Fig. i ) . 



5. Specific Statement of Results and Description 

 OF Experiment. 



(a) Result Specifically Stated. — The general result 

 stated in specific terms is this: Under certain conditions 

 when a. c. is the artificial division plane, F. P. is inhibited 

 and entirely disappears, and a.f.p., which in the normal 

 course of things would have become the tail of A. T., under 

 the experiment becomes added to P. T., to make its head; 

 i. e., the derivative worms become A. Y. — a.f.p., and 



p.r.-\-a.f.p. 



This result follows whatever be the distance of a. c. from 

 F.P. 



The time at which a. c. is made relative to the state of 

 advancement of F. P. is a matter of much greater impor- 

 tance than is its position. As would naturally be expected, 

 the less advanced F. P. is, the greater the likelihood that it 

 will be inhibited ; and after it has reached a certain stage of 

 advancement its inhibition is impossible. The latest stage 

 of F. P. that has been inhibited is that marked by the 

 beginning of the " ciliated pits," but in only a few cases 

 advanced to this extent has success been achieved. Oper- 

 ated on at this stage, the normal fission usually proceeds to 

 completion. 



