The neural groove at this stage constitutes approximately 70% of the total embryonic length. Müller F., O'Rahilly R., (1983)
The neural folds are well developed but still open. They approach each other to the greatest extent at the hindbrain/midbrain junction.
A mesencephalic flexure, particularly marked in the more advanced embryos e.g. 1878, demarcates the mesencephalon. Rostral to the midbrain lies the prosencephalon and caudally the rhombencephalon. O'Rahilly R., Müller F., (1994) pp. 35-42.
The neural folds in this region become elevated as mesenchyme beneath increases and the dorsal aortae and aortic arches enlarge. As the mesencephalic flexure becomes less acute the prosencephalon bends ventrally. The prosencephalic folds can be divided into an optic part (D1) and a postoptic part (D2)
The terminal notch which is present in a proportion of embryos (8/13), according to Müller F., O'Rahilly R., (1985) may be used to identify the future telencephalic area.
The rostral neuropore closes bidirectionally from the rhombencephalon to the mesencephalon and from the chiasmatic plate to the roof of D1. The closure begins in the mesencephalon at about 13 pairs of somites, the neuropore does not disappear finally until the 20 somite stage i.e. in 4 of 18 embryos of this stage studied by O'Rahilly R., Müller F., (1994) p334. Also provided on pp.58-59 is a more detailed description of closure at the terminal lip with photographs from the 20 somite embryo Carnegie No. 6784.
As the rostral neuropore closes, the terminal notch (the point of fusion of the lateral walls of D1) moves towards the mesencephalon, forming a terminal lip, rather than terminal notch. The fusion of the neural folds in this area creates the embryonic lamina terminalis (part of the future commissural plate). This is the basis of the definition of the neural folds rostral to the optic sulcus as 'future telencephalon' e.g. in the 14 somite embryo Carnegie No. 470 described by Bartelmez G.W., Evans H.M., (1926)
However, in the 20 somite specimen Carnegie Embryo No. 2053, the division of the forebrain into diencephalon and telencephalon cannot be made according to Davis C.L., (1923). Presumably this is due to the lack of histological evidence as well as lacking identification of a telencephalon specific structure
The rostralmost division of the diencephalon, D1 contains the optic pits which are present as laterally situated outpocketingís of the forebrain. According to O'Rahilly R., Müller F., (1987).p.119, optic pits develop from the optic sulci at about 14 somitic pairs. When the rostral neuropore closes, at about the 17 somite stage, the pits may be referred to as optic vesicles.
A deepening sulcus dorsally defines the optic vesicle.
The caudalmost part of the diencephalon, D2, often demarcated by a post-optic recess, will later form the thalamic region.