TWINNING AS A MODE OF REPRODUCTION 213 



1. A gametic non-axiate reproduction. — Little need be 

 said about the modes of reproduction included in this 

 category. They have sometimes been considered as 

 cases of internal budding, because groups of cells are 

 cut off instead of single cells as in gametic reproduction. 

 There are probably various gradations between agametic 

 and gametic reproduction. In the case of polyembryony 

 in the parasitic hymenoptera it seems to be true that at 

 first small groups of cells become isolated from the 

 embryo or polygerm and, after two or three generations 

 of somewhat gross fragmentation of the polygerm, single 

 cells become isolated and develop into the definitive 

 sexual embryos. Thus polyembryony seems to involve 

 both agametic and gametic non-axiate reproduction. 



2. Gametic reproduction.— Th.Q characteristic feature 

 of this type of reproduction is that single, more or less 

 specialized, cells (gametes) are formed which, on isolation, 

 are capable of reproducing a whole organism. The 

 period at which gametes may be isolated varies greatly 

 in different groups of animals. When the isolation 

 takes place in an embryonic, larval, or juvenile stage, 

 we speak of the condition as paedogenesis. One of the 

 most interesting life-cycles, involving very early paedo- 

 genesis, is that of the liver fluke. In this species of 

 parasitic flatworm the very young ciliated larva, the 

 miracidium, bores its way into the soft tissues of a snail 

 and there grows into a bag-like vesicle called the sporocyst. 

 The sporocyst produces within its cavity a number of 

 cells which behave exactly like parthenogenetic gametes; 

 for each cell undergoes maturation and goes through 

 a regular process of cleavage resulting in a larva of a 

 different sort, called a redia. These redias in turn 



