16 



INTRODUCTION TO ZOOLOGY 



CHAP. 



base ; it then breaks away and swims off, but finally settles 

 down, probably at some distance from its parent, and becomes 

 attached by its basal end to some submerged object. It then 

 loses its recently acquired cilia and its base elongates, form- 

 ing a long contractile stalk ; — thus it acquires its adult form. 

 Besides this simple method of reproduction, there is 

 another, which is not in the first place a process of multi- 

 plication, but a fusion of two individuals into one ; this 

 fusion, which is known as conjugation, results, however, in 

 increased vital activity and more rapid multiplication by 

 fission (compare Amoeba, page 9). 

 Coniueation ^^ * preliminary to the process of the con- 



jugation of 

 Vorticella bell divides 



two individuals, we find that a 

 into two unequal parts, and that 

 before separating from the 

 parent stalk, one of these 

 daughter bells may divide 

 again into from two to eight 

 parts. In either case, the 

 smaller zooid or zooids so 

 formed develop a basal circle 

 of cilia, and become detached, 

 free-swimming, barrel-shaped 

 bodies (Fig. 7). 



After swimming for some 

 time, one of these small bodies 

 attaches itself near the base of 

 the bell of a stationary Vorti- 

 cella, and, after a complicated series of changes has taken 

 place in tlie nuclei of the two, it is absorbed into the 

 stationary bell, complete fusion of the protoplasm and of 

 the smaller nuclei of the two bodies taking place (the large 

 nuclei break up .and disappear before fusion).i 



Such a process of permanent fusion is known as conjugation, 

 and the conjugating bodies are called gametes. Since they 

 are of two sizes, they are distinguished as the macrogamete 

 and the microgamete, but in the case of Vorticella it is to be 

 noted that the macrogamete is merely the ordinary Vorticella 

 bell, whilst the microgamete is a specially differentiated body. 



' See Saville Kent's Manual of the Infusoria, vol. 

 (London, 1882). 



(After Saville 



A bell dividing to form one daughter 

 bell and several microgametes (m); 

 Gi, early stage in the fusion of a 

 microgamete (m), and a macrogamete. 



ii. pp. 669 and 670 



