EXTERNAL FACTORS OF REGENERATION IN ANIMALS 29 



piece, and a new pharynx appears at the border between the old and 

 the new parts. If one of the pieces is fed at intervals, it is found 

 that the new part grows more rapidly than does the new part in the 

 piece without food. The old tissue in both pieces has shortened 

 somewhat after the operation, and has also decreased somewhat in size 

 as the first new material developed along the cut-side, but in the 

 piece that is fed the old half begins to increase again until it reaches 

 its former size, and may even surpass the latter. A large full-sized 

 worm is produced from this piece, as shown in Fig. 13^, B, C, D. In 

 the starved piece the old part continues to grow small, due to the lack 

 of food and also to the increase in the new side. This increase takes 

 place very slowly, but ultimately a small symmetrical worm may be 

 produced, as shown in Fig. 13.}, E, F, G. It will be seen that the 

 starved piece needs to produce relatively less and less new material 

 in order to become symmetrical, because as the old material diminishes, 

 the pharynx comes to lie nearer to the middle line. 



EFFECT OF LIGHT ON REGENERATION 



Although few experiments have been made to test the effect of 

 light on regeneration, it is certain that in many cases light has no 

 effect on the process, neither as to the quality nor the quantity of the 

 result. In one form, a tubularian hydroid, Eudendrium raccmosum, 

 it has been shown by Loeb that the regeneration of the hydranth 

 takes place only when the animal is exposed to light. When a 

 colony of eudendrium is brought into the laboratory and placed in an 

 aquarium, the hydranths soon die ; but if the colony is kept in a lighted 

 aquarium, new hydranths are regenerated in a few days. If, on the 

 other hand, the colony is kept in the dark, new hydranths do not 

 appear ; but if it is brought back again into the light the hydranths 

 appear. In one experiment one lot of pieces was kept in diffuse day- 

 light, and another lot in the dark. The former produced fifty new 

 hydranths in a few days; those in the dark had not made any 

 hydranths after seventeen days. They were then brought into the 

 light, and in a few days several hydranths had developed on each 

 piece. 



Loeb also tried the effect of different colored light on the regen- 

 eration of eudendrium. Dishes containing pieces of the hydroid were 

 put into a box that was covered by colored glass plates. Pieces sub- 

 jected to dark red and to dark blue light gave the following results. 

 The old hydranths, as is generally the case, were absorbed in the 

 course of three days. The first new hydranths appeared in the blue 

 light on the fourth day, and during the following days the hydranths 

 in this lot steadily increased. Eight days after the beginning of the 



