It may be asked if the placenta does not form a barrier to the passage of poisons into the circulation of the fetus; this would seem to be demonstrated by mercury, which was always found in the placenta and never in the fetal organs.

In poisoning by lead and copper the accumulation of the poison in the fetal tissues is greater than in the maternal, perhaps from differences in assimilation and disassimilation or from greater diffusion.

Whilst it is not an impermeable barrier to the passage of poisons, the placenta offers a varying degree of obstruction: it allows copper and lead to pass easily, arsenic with greater difficulty.

The accumulation of toxic substances in the fetus does not follow the same law as in the adult.

They diffuse more widely in the fetus.