XII METAMORPHOSIS 133 



The result of conjugation is strikingly different in the three 

 cases already studied : in Heteromita (p. 41) the two gametes 

 unite to form a zygote, a motionless body provided with a 

 cell-wall, the protoplasm of which divides into spores : in 

 Paramoecium (p. 114) no zygote is formed, conjugation being 

 a mere temporary union : in Vorticella the zygote is an 

 actively moving and feeding body, indistinguishable from an 

 ordinary individual of the species. 



Vorticella sometimes encysts itself (Fig. 25, h^), and the 

 nucleus of the encysted cell has been observed to break up 

 into a number of separate masses, each doubtless surrounded 

 by a layer of protoplasm. After a time the cyst bursts, and 

 a number of small bodies or spores (h^) emerge from it, each 

 containing one of the products of division of the nucleus. 

 These acquire a circle of cilia (h^), by means of which they 

 swim freely, and they are sometimes found to multiply by 

 simple fission (h"*). Finally, they settle down (h^) by the 

 end at which the cilia are situated, the attached end begins 

 to elongate into a stalk (h*'), this increases in length, the 

 basal circlet of cilia is lost, and a ciliated peristome and 

 disc are formed at the free end (h"). In this way the 

 ordinary form is assumed by a process of development 

 recalling that which we found to occur in Heteromita (p. 43), 

 but with an important difference : the free-swimming young of 

 Vorticella (h^), to which the spores formed by division of 

 the encysted protoplasm give rise, differ strikingly in form 

 and habits from the adult. This is expressed by saying 

 that development is in this case accompanied by a 7neta' 

 morphosis, this word, literally meaning simply a change, being 

 always used in biology to express a striking and fundamental 

 difference in form and habit between the young and the 

 adult ; as, for instance, between the tadpole and the frog, 

 or between the caterpillar and the butterfly. It is obvious 



