VORTICELLA 51 



locomotor cilia, as mentioned above, and breaks away from the 

 stalk as a free-swimming individual. It soon settles down, de- 

 velops a new stalk, and, when it has grown to normal size, may 

 reproduce again. 



Conjugation. The process of conjugation in Vorticella is some- 

 what different from that noted in Paramecium, in which two indi- 

 viduals of the same size (isogamous) fuse temporarily for the 

 purpose of interchanging nuclear material, and then separate. In 

 Vorticella there is a permanent fusion of a small, specialized free- 

 swimming individual, the 'microgamete,' with a normal-sized, 

 attached individual, the 'macrogamete.' This process is known 

 as anisogamous conjugation. We may regard the two types as 

 constituting the male and female elements respectively. The 

 main features of conjugation in Vorticella are as follows : 



(1) The 'female macrogamete' is, to all appearances, a typical 

 Vorticella. 



(2) The 'male microgamete' is a small atypical cell without a 

 stalk. It is formed by the rapid division of an apparently typical 

 Vorticella into four small cells, all of which quickly separate and 

 swim away to seek a macrogamete. 



(3) When a microgamete comes into contact with a macro- 

 gamete, it fuses with it permanently. The place of attachment 

 is generally at the apex near where the stalk is attached. Fertiliza- 

 tion with synkaryon formation occurs through the fusion of the 

 nuclear material of the microgamete and of the macrogamete. 

 Thus the essential features of the nuclear changes in the conjuga- 

 tion of Vorticella agree with those of Paramecium. 



4. Adaptation 



It has been noted above that free-swimming, stalkless Vorti- 

 cellae, which bear an additional band of cilia for locomotion, are 

 occasionally found. These individuals are regularly formed at the 

 time of reproduction, but it is known that they may also occur 

 when the environmental conditions become unsuitable. Appar- 

 ently the development of such free-swimming individuals is to be 

 regarded as an adaptive measure by means of which the possi- 

 bility of finding better living conditions is supplied. 



REFERENCES 

 Consult the list given for the previous chapter (p. 47). 



