data “turned OFF”, so that only the geometry of the otherwise isotropic homogeneous foetal heart affects the dynamics of the vortex. Here, following the geometry of the heart, the organising filament of the initial vortex [ also breaks into the two short pieces, each of which also finds its own synchronous pathway similar to the beginning of the raw DT-MRI simulation shown in Fig. \[3D\_iso\]. However, this time, after a few rotations, the two re-entries find their end in their also almost synchronous termination of the filaments in the base region of the foetal heart, see Fig. \[3D\_shaven\_iso\].]{}

Fig. \[3D\_shaven\_aniso\] shows the *anisotropic dynamics* of the excitation vortex, that is with the fiber orientation data “turned ON”, so that both the anatomically realistic geometry and the anisotropy of the otherwise homogeneous foetal heart affect the dynamics of the initial vortex. Here, the anisotropy of the heart [ also causes the significant transient distortion of the organising filament of the initial vortex, followed by its fast drift towards the apex, and the ultimate termination at the AV border before a completion of a single rotation, very similar to the raw DT-MRI simulation shown in Fig. \[3D\_aniso\]. However, this time, without the 