REGULATION AND FISSION IN PLANARIA. 121 



to it affords a remarkable variety of results in these small pieces. 

 Five different methods of regulation occur: (i) The piece may 

 form a normal animal ; (2) it may be fully used up in the forma- 

 tion of a head and anterior region without a pharynx or post- 

 pharygneal region (Fig. 12). In such "cases a dorsal thickening 

 occurs in the posterior region (Fig. 13, optical section in median 

 plane ); (3) the piece may go entirely to the formation of a posterior 

 region without head and with or without pharynx (Fig. 14). Such 

 pieces bear a dorsal thickening in the anterior region (Fig. 15); 

 (4) the piece may form a head at each end (Fig. 16). In these 

 cases there is a dorsal thickening in the middle (Fig. 17); (5) 

 the pieces may form a tail at each end (Fig. 18). Such pieces 

 acquire after two or three weeks the outline shown in Fig. 19 in 

 consequence of the opposed activity of the two posterior ends. 



The cases 3 and 4 have been described by Morgan and others 

 but so far as I am aware cases 2 and 5 have not been recognized 

 as what they actually are, though they have undoubtedly been 

 seen by many observers and grouped with abnormalities. The 

 pieces from the postpharyngeal region almost without exception 

 produce normal animals. Occasionally a case of double hetero- 

 morphic heads like Fig. 16 occurs very near the extreme pos- 

 terior end of the body, often in next to the last piece. 



Thus the middle region as compared with the anterior and 

 posterior regions appears to be indifferent ; pieces from it may 

 form single or double anterior parts, or single or double poste- 

 rior parts, or normal animals. The different possibilities do not 

 occur in any definite order ; a single or double " tail " is as likely 

 to follow a double " head " as is a normal animal and vice versa. 

 Neither does the presence of the old pharynx play any part in 

 the production of these various results, for the region where they 

 occur most frequently begins some distance anterior to the pharynx 

 and ends some distance posterior to it. 



I believe that we must consider this region as physiologically 

 indifferent, /. c., neither ''anterior" nor " posterior " in the func- 

 tional sense to any marked degree. In consequence of this indif- 

 ference the actual course of regulation must depend on slight 

 chance internal differences in the pieces. If one end becomes 

 physiologically dominant the piece may give rise to a single 



