in the middle through the ventricles of the heart. ]{}

Fig. \[3D\_iso\] shows the *isotropic dynamics* of the excitation vortex, that is with the fiber orientation data “turned OFF”, so that only the geometry of the otherwise isotropic homogeneous foetal heart affects the dynamics of the vortex. It is known that the chosen FHN kinetics parameter values ${\alpha}=0.3$, ${\beta}={0.71}$, ${\gamma}=0.5$ produce rigidly rotating vortex with the positive filament tension [@ft], which, depending on the topology, either collapses or straightens up between the opposite boundaries of the excitable medium. [[In the 3D anatomically realistic isotropic simulations of the foetal heart, the anatomically realistic boundaries of the heart cause drift of the excitation vortex, and, depending on the initial position of the vortex filament, vortices with the positive filament tension tend to collapse. However, there exist initial locations of the excitation vortex, which although result in the drift of the vortex, still do not lead to the expected collapse of the vortex with positive filament tension. One of such outcomes is shown in the Fig. \[3D\_iso\]. Here, following the geometry of the heart, after a very short transient, the initial vortex filament breaks into the two short pieces, each of which finds 