INTERNAL FACTORS OF REGENERATION IN ANIMALS 43 



LATERAL REGENERATION 



Since the most familiar cases of regeneration are those that take 

 place at the anterior and posterior ends, we not unnaturally come to 

 think of polarity as a phenomenon connected only with the long 

 axis of the animal ; but there are also many cases of lateral regenera- 

 tion in which a similar relation can be shown. In such a case as the 

 regeneration of the leg of a salamander, or of a crab, we find 

 instances of lateral regeneration, but since the development takes 

 place in the direction of the long axis of the leg, the polarity of the 

 leg may be thought of as substituted for that of the body. In other 

 animals, however, the regeneration is strictly lateral. I have found 

 that if the anterior end of an earthworm, or even of lumbriculus, is 

 split lengthwise in halves, and then one of the half-pieces is removed, 

 the missing half is replaced by the half left attached to the rest of 

 the worm. Trembley split a hydra lengthwise into two pieces, and 

 each piece bent inwards to make a new tubular body. Bickford, 

 Driesch, and I have obtained similar results with pieces of the stem 

 of tubularia. 



In planarians which have a flat, broad body, lateral regeneration 

 takes place readily. If a worm is split in two along the middle line 

 of the body (Fig. 13^, A), each half regenerates the missing half. 

 This is brought about by the development of new tissue along the 

 cut-side, and the extension into the new part of outgrowths- from the 

 digestive tract. Lateral regeneration also takes place if the worm 

 is split lengthwise into two unequal parts. In this case the larger 

 piece produces new material along the cut-side, and into this new 

 part the branches of the old digestive tract extend. The smaller 

 piece also produces new material along the cut-side, a new pharynx 

 appears along the line between the old and the new tissue, and a new 

 digestive tract is formed out of the remains of the old one (Fig. 

 19, a, b, c). New branches grow out of the fused part into the 

 new tissues at the side. The new worm that develops from a piece 

 that is less than half the width of the old worm is about as wide as 

 the piece that was cut off, for what is gained at the cut-side is lost 

 in the old part. The piece loses in length also during regeneration. 

 If the new worm is fed, it increases in size, gaining in breadth both 

 on the old side, as well as on the new side, and in time it becomes a 

 full-grown, symmetrical worm. 



In the formation of the new part in these cases of lateral regenera- 

 tion it is not difficult to understand how some of the old organs, as 

 the digestive tract, grow out laterally into the new part ; but it is 

 more difficult to see how longitudinal organs, such as the nervecord 

 and genital ducts, are formed anew. Bardeen, who has examined the 



