AGAMIC REPRODUCTION AND REJUVENESCENCE 125 



slightly stimulated when creeping about, or in some cases without 

 any stimulation from external sources being apparent, the posterior 

 region suddenly attaches itself tightly to the underlying surface 

 by its margins, using the ventral surface as a sucking disk, while 

 the anterior zooid continues to creep, and when it feels the resist- 

 ance to forward movement it exerts itself violently to pull away. 

 The consequence of this lack of co-ordination between the two 

 regions is that the body just anterior to the attached region be- 

 comes more and more stretched and finally ruptures, and the 

 posterior region is left behind. Fig. 31 shows an animal in the act 

 of fission. The anterior zooid bearing the head is endeavoring to 

 move forward, and the posterior zooid has attached itself firmly 

 to the surface on which the animal was creeping. In many cases 

 the posterior region of the first zooid becomes stretched into a long, 

 slender band, and even then, particularly in large old animals 

 where the tissues seem to be tougher and rupture less readily, the 

 anterior zooid often apparently becomes exhausted and ceases to 

 exert itself, or else the posterior zooid is torn from its attachment 

 to the substratum or releases itself before the connecting parts are 

 ruptured. Such failures of fission are very common in the larger, 

 older animals. Fission can also be prevented by keeping the ani- 

 mals on surfaces to which they cannot attach themselves firmly, 

 e.g., in vaseline-lined dishes. 



After separation the smaller posterior piece undergoes reconsti- 

 tution into a new animal of small size in exactly the same manner 

 as do pieces cut from the body, and the anterior zooid develops a 

 new posterior end in which one or more new zooids may arise. In 

 Planaria dorotocephala this is the only form of reproduction which 

 has been observed in nature during a period of observation covering 

 some ten years, but in the laboratory, animals which have been 

 prevented from undergoing fission have become sexually mature 

 in a few cases. 



THE OCCURRENCE OF REJUVENESCENCE IN AGAMIC REPRODUCTION 



IN Planaria dorotocephala AND P. maculata 



Since a greater or less degree of rejuvenescence occurs in the 

 reconstitution of pieces of Planaria (see chap, v) and since the 



