RECONSTITUTIONAL PATTERNS IN EXPERIMENT 367 



Certain cases of bipolarity and multipolarity in planarians are of special 

 interest. One or two heads occasionally appear at the anterior end of a 

 longitudinal split extending over most of the body length from the pos- 

 terior end (Fig. 124, A, B).^ Probably a certain degree of physiological 

 isolation is concerned in the development of these heads; such isolation 

 results when there is little direct nervous connection with more anterior 

 regions. In these cases the longitudinal split is between the nerve cords; 

 consequently, only cut transverse commissures can be present at the sur- 

 faces of section. If they are present, they may play a part in localizing 

 head development. As regards the new tissue developing on the longi- 

 tudinal cut surfaces, the heads are anterior, perhaps because gradient- 

 level and reactivity are higher there, though head development may occur 

 at more posterior levels of a longitudinal split, as will appear below. 



Cruciate forms with a single head at the anterior end of the longitudinal 

 split (Fig. 124, B) and a variety of multipolar forms resulting from the 

 same operation have been described for Dugesia tigrina by Silber and 

 Hamburger (1939). One of these, resulting from a second longitudinal 

 split of each part separated by the first split, is shown in Figure 124, C. 

 This form became cruciate after the first split; but after the second, heads 

 regenerate from the longitudinal cut surfaces of the two lateral posterior 

 ends. Three distinct heads and five other pairs of eyespots in early stages 

 are present on these surfaces. The ventrodorsality of these heads coincides 

 with that of the body, and their bilateral form is evidently due to the 

 fact that each one represents a localized region of outgrowth with a ven- 

 trodorsal pattern. The nervous system in these lateral posterior ends must 

 be undergoing or have undergone extensive reorganization; but whether 

 parts of it are concerned in localizing the outgrowths which become heads 

 does not appear. In some species of planarians more readily than in others 

 a head may regenerate from a lateral region partly separated from the 

 body by an oblique section extending posteriorly and with its anterior 

 end removed to provide an anterior cut surface; from this the head de- 

 velops. Provided the longitudinal separation persists, an indefinite num- 

 ber of heads along the sides of the original body, with more or less induced 

 reorganization posterior to each, may be produced. This is essentially a 

 variation of the experiment of Figure 135, ^ (p. 400). 



As regards establishment of new dominant regions, determination of 



* Van Duyne, 1896; Morgan, 1900a; Goetsch, 1921, 1922, 1928; Keil, 1924; Beissenhirtz, 

 1928; Silber and Hamburger, 1939. _^ 



