ZooL— Vol. II.] RITTER-CONGDON—STENOSTOMA. 369 



Soon after this, careful examination, particularly by com- 

 paring measurements made now with those made at the 

 time of the operation, discovers that the cell masses (c. m.) 

 have changed position somewhat; and from this on it 

 becomes obvious that they are slowly moving toward the 

 anterior end of the piece. This migration continues until 

 they have reached a position which, relative to the whole 

 of f. -p., would correspond with that of the brain in a nor- 

 mal worm, and here they become fixed and form the brain 

 of the new worm, P. T. -j- a. f. f. A curious fact about 

 this migration is that the left cell-mass appears to always 

 advance more rapidly than the right, (see figures, PL XVII). 



We have no evidence that the ciliated pits migrate for- 

 ward, as do the ganglionic cell-masses, when they are 

 present in P. Y. at the time of section. On the contrary, 

 without having reached positiveness on this point, it appears 

 that the original foundations disappear, and that new ones 

 form in a. /.p., the head of the new worm. The incipient 

 changes that had set in in ectoderm and gut-wall likewise 

 gradually disappear. 



By the time the cerebral ganglia reach their destination, 

 the anterior end of the worm has become filled, within 

 the ectoderm, by a loosely arranged, confused mass of cells 

 within which the brain becomes more or less hidden. From 

 this incoherent mass the pharynx seems to take its origin; 

 but precisely what happens here is impossible to deter- 

 mine on the living animal. That, however, the new mouth 

 at least is formed by breaking through the ectoderm of 

 the piece (a. f. -p.) there can be no doubt. 



((f) Behavior when the Artificial Section is -p. c. — This 

 case has never resulted in the inhibition of P. P., though 

 it must be said that fewer experiments have been per- 

 formed here than in the other case, i. e., with a. c. 

 as the division. This operation has, however, a marked 

 and interesting influence on the further course of events. 

 While a. c. frequently has the effect, when it does not in- 

 hibit P. P., of hastening it perceptibly, p. c. on the con- 

 trary appears always to exercise a retarding influence on 



