DOUBLE STRUCTURES 135 



It then begins to grow faster on one side (Fig. 21, b\ and a head 

 appears in this region with its axis at right angles to the cut-edge. 

 As the head grows larger the growth is more rapid on one side, and 

 as a result the head is slowly turned forward (Fig. 21, b). This 

 more rapid growth on one side brings the new head finally into its 

 typical position with respect to the rest of the piece. The end result 

 of these changes is to produce a new worm having a typical form. 

 If the oblique cut is made behind the old pharynx, as in Fig. 22, A, 

 the new pharynx that appears in the new material along the cut-edge 

 lies obliquely at first, indicating that the new median line is very 

 early laid down in the new part, and connects the middle line of the 

 old part with the middle of the new head. As the region behind the 

 new head grows larger and broader the pharynx comes to lie more 

 and more in an antero-posterior direction, and finally, when the new 

 part is as broad as the old, 1 the pharynx lies in the middle line of 

 a symmetrical worm. 



These results show that the new growth may even take place 

 more rapidly on one side of the structural median line than on the 

 other, and on that side that must become longer in order to produce 

 the symmetrical form of the worm. Here also we find that a for- 

 mative influence of some sort is at work that regulates the different 

 regions of growth in such a way that a typical structure is produced. 

 The more rapid growth on one side is, however, in this case clearly 

 connected with the relatively smaller development of the organs on 

 that side, and perhaps this same principle may explain all other 

 cases. If so the phenomenon appears much less mysterious than 

 it does when the growth is referred to an unknown regulative factor. 



DOUBLE STRUCTURES 



A structure that is single in the normal animal may become 

 double after regeneration, and in some cases the special conditions 

 that lead to the doubling have been determined. Trembley showed 

 that if the head of hydra is split lengthwise into two parts, 

 each part may complete itself and a two-headed form is produced. 

 If the posterior end of a hydra is split, an animal with two feet is 

 made. It is true that the two-headed forms may subsequently sepa- 

 rate after several weeks into two individuals, and even the form 

 with two feet may lose one of them by constriction, as Marshall and 

 King have shown. Driesch has produced a tubularian hydroid with 

 two heads by splitting the stem partially into two pieces. Each head 

 is perfect in all respects, and although each has fewer tentacles than 



1 If the young worm is fed the new part becomes almost as broad as the old piece, but 

 if the worm is not fed the old part decreases in breadth and the new part does not grow as 

 broad as in the former case. 



