TWINNING AS A MODE OF REPRODUCTION 219 



into fifteen or twenty primary cell masses. Since there 

 is evidently some integration among the cells of these 

 primary cell masses, they may be considered as embryos 

 of the first asexual generation. The primary masses 

 soon lose their unity of organization and break up by 

 constriction and separation into secondary cell masses 

 or embryos of a second asexual generation. This 

 happens once more and tertiary masses or embryos of 

 the third asexual generation are formed. These tertiary 

 embryonic masses then completely disorganize into 

 components which are the equivalent of single germ 

 cells, each of which goes through a typical process of 

 embryonic development to form an adult insect. It is 

 not always possible to trace each insect back to a single 

 cell, but this was done in enough cases to make it practi- 

 cally certain that each one starts from a single cell. 

 Such a cell is therefore essentially a precociously formed 

 germ cell capable of parthenogenetic development. 

 How strikingly like the liver-fluke case this is, and how 

 unlike that of the armadillo, the reader may readily 

 ascertain if he studies the respective life-histories side 

 by side. Unless, therefore, we are prepared to reduce 

 all twinning phenomena to a parity with the condition 

 in the parasitic hymenoptera or even the liver fluke, we 

 should cease to refer to the mode of multiple embryo 

 formation in the armadillo as polyembryony and call it 

 what it really is — ^twinning. 



