he divided the marrow into shapes corresponding in number and fashion to those which the several kinds were destined to wear. And he moulded into spherical shape the plough-land, as it were, that was to contain the divine seed
Plato’s Timaeus commentary identifies the marrow as the original substrate divided into seed forms, each corresponding to the soul-type it is destined to carry, with the brain as the spherical vessel of the divine seed.
, Plato’s cosmology the Timaeus of Plato, 1997thesis