H6 MORGAN. [VOL. II. 



notochord and nerve cord and the tissue immediately around 

 them, and from these the new tail has grown out. I did not 

 discover this until most of my material had been used. After 

 this I cut off the grafted tails nearer the line of union, and 

 although regeneration did not take place so well in several 

 cases, still those that regenerated showed more often both 

 parts contributing to the new tail. The same result followed 

 in a number of cases in the previous operations, and we may 

 now examine how regeneration takes place in such cases. 



In thirteen cases (out of sixty) there was found evidence of 

 a dual or compound character to the new tail. In all cases 

 observed there was no evidence to show that the duality was 

 the result of the tissues being mixed in character either by 

 commingling of the cells (each cell retaining its specific charac- 

 ters) or by a hybridizing of the cells (due to mutual influence). 

 The duality consisted in each part, regenerating cells like itself, 

 so that definite regions of the new tail were made up of one or 

 of the other kind of tissue. For instance, the new tail might 

 be made up above of the slaty-colored tissue of R. sylvatica 

 and below of the yellow tissue of R. palustris. There is no 

 evidence of a shading of one kind of tissue into the other along 

 the line of meeting, but this point would be very difficult to 

 determine positively. There is further no evidence that the 

 two kinds of tissue are any more commingled at the distal end 

 of the tail than at the base. 



In regard to the notochord and nerve cord it is extremely 

 unlikely that the cut would ever pass obliquely through the line 

 of union of the one or of the other, as these structures are very 

 small in cross-section. It is, therefore, probable that in nearly 

 every case the new notochord and the new nerve cord are made 

 up of cells belonging entirely to one component. Further- 

 more, these two structures lie so near together that it is not 

 probable that the cut would pass between them in such a way 

 that the nerve cord at the exposed edge would belong to one 

 component and the notochord to the other component. 



The details of the successful experiments are as follows : 

 On April 14 and 15 nine grafts were made, as shown in Figs. I 

 and 2. On April 16 these were cut off, as indicated in Fig. i, a-a, 



