366 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



surfaces are indistinguishable, one or two heads may develop on the 

 whole surface of section (Fig. 123, 5, C). Apparently, with decrease in the 



B 



Fig. 124, A-C. — Planarian forms with multiple heads after partial posterior longitudinal 

 splitting. A, two heads from anterior regions of sides of split; B, diiplicitas cruciata; C, split 

 twice, the second split dividing each of the parts separated by the first; cruciate after first 

 split, multiple heads from longitudinal surfaces of section of lateral posterior regions after 

 second split {B, C, from Silber and Hamburger, 1939). 



anteroposterior differential, as length of piece decreases, the mediolateral 

 differential plays an increasing part in localization of the head.^ 



5 Child, 1915c, p. 164; Olmsted, 1918; Beyer and Child, 1930. 



