2i8 THE PHYSIOLOGY OF TWINNING 



POLYEMBRYONY AND TWINNING 



It is an unfortunate circumstance that one-egg 

 twinning in the armadillos was described first as a case of 

 polyembryony. The fact that all embryos derived from 

 a single egg were of the same sex strongly reminded the 

 discoverers of twinning in the armadillos of the condition 

 described by Sylvestri and others for the parasitic 

 hymenoptera, where sometimes hundreds of embryos 

 were produced from a single egg and all embryos from 

 any one egg were of the same sex. Since the hymen- 

 opteran condition had with considerable appropriateness 

 been called polyembryony, it was not unnatural that 

 several of us who first studied the armadillo situation 

 should adopt for it too the term polyembryony. At that 

 time we did not fully understand the exact mode of origin 

 of the multiple embryos in either parasitic hymen- 

 op teran or in the armadillo. Now that both conditions 

 are adequately understood, it is clear that the armadillo 

 case is a clear-cut instance of dichotomous twinning, 

 a form of axiate reproduction; while true polyembryony, 

 as seen in such forms as Paracopidomopsis (Patterson, 

 192 1) involves nothing at all equivalent to the processes 

 of twinning, but follows a mode of reproduction essen- 

 tially non-axiate, resembling much more closely the 

 paedogenetic type of reproduction of the liver fluke 

 than it does twinning in the armadillos. In Paracopi- 

 domopsis there are evidences of several embryonic 

 generations that remind one of the several larval genera- 

 tions in the liver fluke, Fasciola. The cleavage period 

 results in a so-called '^morula stage," which loses its 

 axiate relations and becomes a mass of generalized cells 

 called a ''poly germ." This soon becomes subdivided 



