THEORIES OF DEVELOPMENT 249 



is it, I think, a much simpler idea to suppose the structure is some- 

 thing pertaining to the whole and is not the sum total of smaller 

 wholes, but the idea is more in accord with other phenomena. 



We meet here, I think, with precisely the same problem that we 

 meet with in the regeneration of parts of adult animals. If a plana- 

 rian is cut in two lengthwise, along the middle line, each half produces 

 new tissue at the cut-side, out of which the missing half is formed. 

 In this case the old median plane remains, more or less, as the median 

 plane of the new worm, i.e. the structure of the new part is built 

 up on that of the old. Very much the same result follows when the 

 worm is cut longitudinally into two unequal parts. The larger piece 

 retains its old plane of symmetry and adds to the cut-edge a new part 

 that completes the symmetry. The smaller piece also builds up new 

 material along the cut-edge, and a new plane of symmetry is formed 

 between the old and the new parts. Here, also, a median plane 

 is established at the edge of the old material, but in this case the 

 material lay to one side of the old middle line, and this involves the 

 changing over to a large extent of the old material, so that it fits 

 in with the new structures of the new median plane. 



In those forms in which the readjustment takes place entirely in 

 the old part, the change of conditions is more difficult to interpret. In 

 some respects hydra gives us an intermediate condition, but since it 

 is a radially symmetrical instead of bilaterally symmetrical form, the 

 transformation is not so obvious. If a cylindrical piece is cut from 

 the body, and is then cut lengthwise into two half-cylinders, each closes 

 in and makes a cylinder of smaller diameter. A little new tissue 

 may appear along the fused edges, but the missing half is not re- 

 placed, and a new hydra with a body of half size is formed from the 

 piece. It is to all appearances a radially symmetrical form, and we 

 must think, in this case, of the new axis of symmetry as having shifted 

 to the middle of the piece. As yet no similar experiments have been 

 made on a bilateral animal that regenerates by morphallaxis, so that we 

 have nothing to appeal to for comparison with the bilateral egg, but the 

 results, just described for the planarian and for hydra, indicate how a 

 change might take place in pieces of adult animals that would lead to 

 the formation in them of a new symmetrical structure. If we imag- 

 ine a case of this sort, and suppose that after separating a piece from 

 the side the cut-edge closed in and the piece assumed a symmetrical 

 form, it is conceivable that a new plane of bilateral symmetry might 

 soon appear in the middle of the piece owing to the symmetrical form 

 of the piece ; or the new plane of symmetry might slowly shift from 

 the cut-edge toward the middle of the piece, after reaching which the 

 balance or equilibrium would be attained. This statement, it must be 

 confessed, is little more than a supposition, and rests on the unproven 



