Complex sentence: The two notable centres for experiments on the species were Łękno (391 animals altogether) and Popielno (121 animals), while limited experiments were also held in the reserve of Askania Nova in the USSR.
Split into simpler sentences: The two notable centres of experiments on the species were Łękno (391 animals altogether) and Popielno (121 animals). Limited experiments were also held in the reserve of Askania Nova in the USSR.