THE ORIGIN OF NEW CELLS AND TISSUES 2OI 



become ossified, the transition from the old to the new part is at first 

 very sharp. The new tissue, that will make the new cartilages of the 

 new limb, develops as a cap over the cut-end of the old bone. 

 Gotte does not give an explicit statement in regard to the origin 

 of the new cartilage, but his account leads one to suppose that it 

 develops from the old cartilage or from some part of the bone. 

 This is, in fact, the case, as I have observed in preparations of the 

 regenerating leg of Plethodon cincrcns, in which the new cartilaginous 

 tissue comes from the periosteum of the old bone. Gotte shows 

 that two long rods of tissue are formed, that are separate for the 

 greater part of their length. They give rise to the two bones of the 

 lower leg, or forearm, as the case may be. The broken end of 

 the femur or humerus also completes itself by a short cartilaginous 

 cap, which is at first continuous with the two rods just described. 

 The ends of these two rods break up into a series of pieces that 

 form the tarsalia, or the carpalia, and the digits. Two digits are first 

 formed, and the others are added as outgrowths from the side of one 

 of the two rods. It is important to note that the new cartilages are 

 formed, in large part, out of a continuous substratum (or rather of 

 two) which separates into proportionate parts to produce the elements 

 of the new limb. 



The regeneration of the muscles of the limb of an adult animal, 

 plethodon, has been recently worked out by Towle. The leg was 

 cut off in the middle of the forearm. Extensive changes take place 

 in all the muscles that extend across the level of the cut. The old 

 fibres in the lower end of the muscle, i.e. those near the cut-end, 

 disintegrate, and the number of nuclei greatly increases. The divi- 

 sion of the nuclei seems to be direct, each retaining some of the 

 old muscle substance about itself. From some of these cells the new 

 muscle tissue is formed in the new part. Higher up in the forearm 

 the muscle fibres break down to a smaller extent, and still higher up 

 some of the old fibres may remain intact. New muscle fibres are also 

 formed in the old muscle, especially in the region near the cut-end. 



The process of regeneration has not been so fully worked out in 

 any other vertebrates as in those described in the preceding pages, 

 although the regeneration of single tissues or organs in the verte- 

 brates has been extensively investigated. In all such cases it is found 

 that like tissues give rise to like. 



In the planarians it has been found that during regeneration the 

 ectoderm covers the exposed surface, and from it arises the new ecto- 

 derm ; the digestive tract appears to come in part from the old tract 

 and in part from the middle-layer cells ; the nervous system appears 

 also to develop out of the middle-layer cells that are found scattered 

 through the body. These cells seem to form a sort of reserve supply 



