64 



BIOLOGY 



.m 



plex in structure. The first step in the process is the elongation 

 and division of the micronucleus into two parts, one of which 

 comes to lie at each end of the animal; 

 Fig. 23. This is followed by a similar 

 elongation and division of the macronu- 

 cleus. The oesophagus produces a little 

 bud which develops into a new oesopha- 

 gus, and then this and the old one move 

 apart, so that the latter advances to the 

 front part of the body, and the former 

 lies in the posterior part. A new mem- 

 branella develops in the oesophagus. Two 

 new contractile vacuoles make their ap- 

 pearance, one just in front of, and 'one 

 just behind, the middle line of the body. 

 Meantime a constriction has been mak- 

 ing its appearance, which gradually deep- 

 ens, cutting the animal into two parts 

 by a cross division. The two halves 

 thus produced separate from each other 

 and swim away to live an independent 

 life. It should be noted that in this reproduction each of the 

 important parts of the animal divides, so that each of the two 

 new individuals has a part of each organ which the original Para- 

 medum possessed. This multiplication by division may go on 

 almost indefinitely if the animal is properly fed and placed 

 under favorable conditions. Ordinarily it will occur about 

 once in twenty-four hours, although the frequency may vary, 

 becoming greater or less with varying conditions of food and 

 temperature. A continuous reproduction of this kind has been 

 followed for over 2500 successive divisions. Whether it can go 

 on indefinitely if the conditions were favorable is not known. 

 It is known, however, that under ordinary conditions this power 

 of reproduction gradually becomes less and less, and finally 

 tends to disappear altogether. It is believed that in nature 



FIG. 23. PARAME- 

 C1UM IN PROCESS OF 



DIVISION 



m, mouth; mac, macro- 

 nucleus; mic, micronucleus. 



