REGENERATION AND GROWTH 133 



example, we cut off the tail of one fish squarely near the outer end, 

 as shown in Fig. 40, F, i-i , and the tail of a second near the base of the 

 tail, as shown in Fig. 40, F, 2-2, and of a third by an oblique cut that 

 corresponds to a cut extending from the upper side of the cut-edge of 

 the tail of the first fish to the lower cut-edge of the tail of the second 

 fish, as shown in Fig. 40, F, we find that the rate of growth over the 

 first and second tails is about the same as that of the lower side 

 of the third tail. In other words, the maximum rate of growth that 

 is possible for the entire oblique edge is carried out only near the 

 lower edge, and the growth of the rest of the new material is held in 

 check. By means of another experiment a similar phenomenon can 

 be shown. If the bifurcated tail of a young scup {Stenopns chrysops} 

 is cut off by a cross-cut (Fig. 40, G, l-l), it will be found that at first 

 the new material is produced at an equal rate along the entire cut- 

 edge; but it soon begins to grow faster at two points, one above and 

 the other below, so that the characteristic swallow-tail is formed at a 

 very early stage (Fig. 40, //) and before the new material has grown 

 out to the level of the notch of the old tail. If the tail of another 

 individual is cut off by an oblique cut (Fig. 40, G, 2-2), we find, as 

 shown in Fig. 40, /, that at two points the new tail grows faster, but 

 the lower lobe faster than the upper one. 



These results show very clearly that in some way the development 

 of the typical form of the tail influences the rate of growth at different 

 points. The more rapid growth takes place in those regions at which 

 the lobes of the tail are developing. In other words, although the 

 physiological conditions would seem to admit of the maximum rate of 

 growth over the entire cut-edge, this only takes place in those parts 

 that give the new tail its characteristic form. The growth in other 

 regions is held in check. The same explanation applies to the more 

 rapid growth at that part of an oblique cut that is nearest the base 

 of the tail, for by this means the tail more nearly assumes its typical 

 form. 



These results demonstrate some sort of a formative influence in 

 the new part. We can refer this factor at present only to some 

 structural feature that regulates the rate of growth. We find here 

 one of the fundamental phenomena behind which we cannot hope to 

 go at present, although it may not be beyond out reach to determine 

 in what way this influence is carried out in the different parts. This 

 topic will be more fully considered in a later chapter. 



Another illustration may be given from certain experiments in 

 the regeneration of Planaria lugubris. If the posterior end is cut 

 off just in front of the genital pore, as indicated in Fig. 41, new 

 material develops at the anterior cut-edge, and in a few days a new 

 head is formed out of this new material. A new pharynx appears 



