REGENERATION IN PARAMECIUM CAUDATUM. 167 



races were kept under the same conditions as the control. The 

 experiment was begun on March 14, and the first measurements 

 made on the twenty-third showed the same average size in E, E l 

 and E 2 , but E 3 and E 4 were much smaller. The next measure- 

 ments made on the twenty-sixth showed that E 3 had reached the 

 average size, and a week later the small cells of E 4 were fully as 

 large as the control race. 



It is undoubtedly true that a large number of these small 

 fragments, and most of the cells thrown off from the monsters, 

 show signs of extreme feebleness and often die after one or two 

 divisions. It is only those that formed races that are taken into 

 consideration. Small fragments if possessing enough vitality 

 to continue the race will sooner or later become as large as the 

 parent stock. At the present time I have a race descended from 

 a small anterior fragment formed by irregular division. The 

 average size of these cells is slightly larger than that of the pure 

 line from which they arose. 



In this connection it is also interesting to note that after 

 removal of the anterior ends of conjugating cells (Fig. 10, X) 

 when for some reason the fragments differ in size (Fig. n, 

 L and R) the descendants of both halves finally regain the same 



average size. 



GENERAL. 



In considering the phenomena of regeneration and regulation 

 in a unicellular organism one must take into account the fact 

 that we have, in such a form as Paramecium, a highly differen- 

 tiated structure. This differentiation is greater in the anterior 

 end of the cell and diminishes posteriorly. Therefore it is not 

 surprising that less disturbance results from removal of the 

 posterior than of the anterior end. Regeneration follows the 

 removal of the posterior end (Fig. I, Z) in most cases, but this 

 does not mean that the entire end is replaced before fission occurs 

 Such fragments divide before the full size is regained, therefore 

 regeneration amounts to nothing more than the healing over of 

 the wound and the formation of new cilia; the regaining of the 

 original body form is brought about by a gradual process of 

 regulation through growth in volume. After the wound is healed 



