368 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



(h) Outline of Process of Normal Fission. — To gain 

 some notion of what this stage is, a brief statement follows 

 of the order of events in normal fission, as seen by exam- 

 ination of the living worm under moderate magnification. 



The first indication of division is an enlargement of a few 

 cells on each side of the body about two-thirds the length 

 of the animal back from its anterior end. These cells soon 

 come to form a mass of considerable size (figures, c. w., PI. 

 XVII), and occupy the space between the ectoderm and the 

 wall of the gut. These are, according to Ott (1892) , and our 

 observations are to the same effect, the beginnings of the 

 cerebral ganglia, and are due to growths on the lateral nerves 

 of the body. Next follows a clearing up and slight constric- 

 tion of the gut at this point. About simultaneous with the 

 earliest changes in the gut a constriction in the ectoderm 

 takes place ; and slightly later the beginnings of the ciliated 

 pits make their appearance. 



All these changes soon become pronounced, and a little 

 time after the appearance of the ciliated pits the pharynx 

 begins to form. The mouth breaks through before the 

 complete separation of the two worms, and even before the 

 severance of the gut; it does not appear, however, that 

 food is actually taken in through the mouth until the two 

 worms are fully severed. 



From this account it is seen that inhibition of F. P. is 

 possible after it has reached a rather advanced stage; i. e., 

 after changes have begun in all the tissues involved in the 

 division, and the foundations of at least two sets of organs 

 of the head, viz., the cerebral ganglia and the ciliated pits, 

 have been laid. 



(c) Develojf)7nent when F. P. is Inhibited. — After being 

 cut, both pieces remain for a time considerably contracted. 

 They soon, however, assume their normal size and shape, 

 and a. f. almost invariably swims about in a violent way for 

 a time, while -p. -p. generally remains quiet. 



No visible change of any kind, either in F. P. or any 

 other part of the piece, takes place for about twenty-four 

 hours. 



