The basement membranes of the future neurohypophysis and adenohypophysis are no longer in as intimate contact. The hypothalamic floor is without a marginal layer but laterally the hypothalamic cell cord can be found. The mammillary area possesses a thickened ventricular layer, and an indication of an intermediate layer condensation in the hypothalamic walls caudal to the mammillary tubercle. These bilateral condensations are the primordial mammillary nuclei, separated by thin floor lamina. In some embryos it remains in contact with the adenohypophysis, whereas in others the two are separated by mesenchyme.
All embryos have a neurohypophysis with an infundibulum.
In the future hypothalamic area two recesses develop, a supramamillary and inframammilliary recess, demarcating the mammillary area. As the dorsal thalamus grows and the ventral thalamus is displaced laterally, the sulcus medius and the sulcus hypothalamicus fuse to form the adult hypothalamic sulcus. Fig 9E and closely apposed to the adenohypophysis which has now retained only an epithelial cord connecting it to the oral cavity.