GKAFTING AND REGENERATION l8/ 



a dorsal and ventral fin. The muscles of the normal tail have a 

 characteristic V-shaped arrangement with the apex of the V's turned 

 forward, but unfortunately in the new tail the muscles are so 

 irregular that it is impossible to make out their arrangement 

 (Fig. 54, D}. If the new part is in reality a tail, the V's ought 

 to stand in the same way as do those in the major component, and 

 opposed to the V's on the part from which the new material arises. 

 If the new structure is not a tail at all, but a new growth, or even a 

 suppressed trunk, then the V's should stand as in the small part itself. 

 It has not been possible as yet to obtain a decisive case. Harrison 

 obtained one case in which the arrangement of the muscles in the 

 new part seemed to be more as it should appear if the new part is 

 a heteromorphic tail (Fig. 54, D\ Even if this could be shown to be 

 the case, it may be that under the conditions of the experiment the 

 arrangement of the muscles is determined by the use of the tail, 

 although this does not seem very probable. Harrison, after a careful 

 analysis of the question, left it undecided, but seemed more inclined 

 to the view that the result is due to the development of something 

 new rather than a heteromorphic growth. On the contrary I am 

 strongly inclined to believe that the latter is the true explanation. In 

 another way I have been able to bring about the development of the 

 same structure. A small triangular piece is cut from the upper part 

 of the tail, as indicated in Fig. 56, A, one point of the triangle passing 

 through the notochord, or even through the aorta. If the cut-surfaces 

 are kept apart for a few hours, until the exposed end has been covered 

 over by ectoderm, they may not unite afterward, and two exposed 

 surfaces are left, one at the distal end of the base of the tail, and the 

 other at the proximal end of the outer part of the tail. The latter 

 surface corresponds to that in the grafting-experiment. Regenera- 

 tion may take place from the two surfaces ; both new parts seem to 

 be exactly alike, and both resemble a regenerated tail. The one from 

 the proximal surface of the outer part of the tail contains a notochord, 

 nerve-cord, connective tissue, pigment cells, and muscle tissue (Fig. 56, 

 B\ The arrangement of the muscle fibres is generally very irregular, 

 and the characteristic V-shaped arrangement cannot be detected. 



In only a few cases have attempts been made to unite two eggs 

 or two very early embryos, although there are a few casual observa- 

 tions * in which such a fusion has been observed. The problems that 

 arise in connection with the union of two eggs are full of interest. 

 Each egg has the power of producing an embryo of normal size. If 

 two eggs are united into one, will a single giant organism result, or 

 two organisms ? If the former, we must suppose that a new organi- 

 zation is formed of double size. Whether an upper limit of organiza- 

 i Metschnikoff ('86), Herbst ('92). 



