COl^JTROL OF HEAD-FORM IN PLANARIA 123 



1 and 2, lie, He, 11/), unless the concentration is too high 

 (table 1, III d, III e, HI/). This shift in head-frequency is, 

 however, almost entirely a shift from teratopthalixiic to nor- 

 mal which represents much less physiological change than a 

 shift from anophthalmic or headless to teratopthalmic. In 

 other words, the change produced by KCN in the relation 



— in these pieces is much less than in the short period c- 



rate y 



pieces of the first zooid in tables 3 to 5. The extreme posterior 

 piece (/, tables 1 and 2) has normally a head-frequency of 100 

 per cent or nearly and this cannot be shifted upward by cyanide 

 and, as in the a-pieces short periods have little effect in shifting 

 it downward while long periods produce a marked shift down- 

 ward. 



In short, all the facts are in complete agreement, the differ- 

 ences in metabolic condition, the differences in head-frequency 

 and the different effects of low concentrations of cyanide in 

 both short and long periods on pieces from different regions of 

 the body all afford evidence for the same conclusion, viz., that 



head-frequency = and there are no conflicting or con- 

 rate y 



tradictory data. 



Another possible factor which may play a part in determining 

 rate x must, however, be mentioned, though at present I see 

 no way of demonstrating experimentally whether it is con- 

 cerned or not. It may be that the differences in metabolic 

 rate at different levels of the body, i.e., of the axial gradient 

 (Child, '12, '13 b, '13 c, '15 b), determine intrinsic differences 

 in the rate of reaction of the cells which constitute the region 

 .T of a piece and which give rise to a head when a head is formed. 

 The experimental difficulty in testing this possibiUty lies in 

 the fact that the region x is associated with a region y which 

 inhibits this reaction to a greater or less degree. The only 

 possible way of testing it would be to cut pieces so short that 

 they consist only of the region x, i.e., so short that the whole 

 length of the piece is involved in head-formation. It is possible 

 to approach this condition in this species of Planaria but not 



