its own synchronous perpetual pathway in the “isotropic” foetal heart, resulting in the seemingly perpetual cardiac re-entry, which failed to self-terminate within the extended simulation time, see Fig. \[3D\_iso\].]{}]{}

Fig. \[3D\_aniso\] 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.

In the 3D whole heart MRI-based simulations shown in the Fig. \[3D\_X\_iso\]

![image](Fig5small.pdf)

and Fig. \[3D\_X\_aniso\],

![image](Fig6small.pdf)

a counter-clockwise excitation vortex was initiated by the phase distribution method [@chaos], with the initial position of the transmural vortex filament (yellow line) at the prescribed location [*along the $y$ axis*]{} at $x_0=40, z_0=60$[, that is *perpendicular* to the initial orientation of the vortex filament shown in Fig. \[3D\_iso\] and Fig. \[3D\_aniso\]. It can be seen in Fig. \[3D\_X\_iso\] isotropic, and in Fig. \[3D\_X\_aniso\] anisotropic 3D simulations, that at $t=0$, there was identical intial location of the filament of the excitation vortex: that is transmurally, roughly in the middle through the ventricles of the foetal heart, and *perpendicular* to the initial orientation of the vortex filament shown in Fig. \[3D\_iso\] and Fig. 