Assertion: african clawed frog's number | /r/HasProperty | difficult to determine
Facets:
value: to the present
type: TEMPORAL
value: since 1970
type: TEMPORAL
Source sentences:
text: While the exact numbers of Xenopus laevis used in research world-wide are difficult to determine, since 1970 to the present, there have been >25,000 research reports using Xenopus .