DOUBLE STRUCTURES 141 



heads do not appear unless the cut extends far forward, but if the 

 division extends into the region between the two eyes there may be 

 formed, as I have found, a single eye on each side that makes a pair 

 with the old eye of that side (Fig. 44, A). It is evident in this case 

 that each head has completed itself on the cut-side, the completion 

 including the eye and the side of the head also with its " ear-lobe." 

 The result, in this case, is the same as though the pieces had been 

 completely cut in two. If the cut does not extend quite so far for- 

 ward there are usually formed one or two heads near the angle, each 

 with a pair of eyes and a pair of ear-lobes (Fig. 44, C). Sometimes 

 a single head develops in the angle itself (Fig. 44, Z>), and it is diffi- 

 cult to tell whether it belongs to one or to the other side, or whether 

 it is common to both sides. Van Duyne spoke of the double and 

 single head of the latter kind which he obtained as heteromorphic 

 structures in Loeb's use of the term. According to this definition, 

 heteromorphosis is the replacement of an organ by one that is morpho- 

 logically and physiologically unlike the original one, but this statement 

 has been made to cover a number of different phenomena. The 

 examples of heteromorphosis that Loeb gives by way of illustration of 

 the phenomenon are : the production of a hydranth on the aboral end 

 of tubularia, and the formation of roots in place of a stem in anten- 

 nularia, etc. The formation of the heads in the angle in planarians 

 does not appear to me to belong in this category. It seems rather 

 that the phenomenon is of the same sort as the formation of a new 

 head at the side of a longitudinal piece, and if so the new heads in 

 the angle are, therefore, in their proper structural position for new 

 heads belonging to the posterior halves. Even if it should prove 

 true that a single head may develop exactly in the angle itself, and 

 belong to both sides, it can be interpreted by an extension of the same 

 principle. 1 The position of this median head turned backward sug- 

 gests an obvious comparison with the production of the heteromor- 

 phic head in Planaria Ingubris, but a closer examination will show, 

 I think, that the two cases are different. The heteromorphic head 

 is produced only when the head is cut off close behind the eyes. If 

 cut off slightly behind this region, a posterior end is generally 

 formed. But in the worms split lengthwise the head in the angle 

 may be formed at a level much farther posteriorly than the eyes. If 

 the split extends into the head, then the eyes that develop are the sup- 

 plements of those of the old part. Our analysis leads, therefore, to 

 the conclusion that the heads, or parts of heads, in the split worms 

 are not heteromorphic structures but supplementary heads. 



1 A parallel case is found when a piece partially split in two at the anterior end (Fig. 24) 

 produces one or two heads on each half, according to the extent of fusion of the new mate- 

 rial that goes to form the new head or heads. 



