The neural plate region is apparently histologically distinguishable from the somatic ectoderm at this stage. O'Rahilly R., Müller F., (1981)
In amphibians the somatic ectoderm expresses a protein, Epi-1 that is absent in the presumptive neuroectoderm before it is supposedly induced by chordamesoderm. This suggests that the future neural ectoderm is destined to become neural before it is histologically recognised as such. Gilbert S.F., (1994) p.612
In about 25% of embryos at this stage a neural groove is visible in the plate O'Rahilly R., Müller F., (1981)
It was noted that only in embryos longer than 1mm and wider than 0.6mm with at least a 0.3mm primitive streak and 0.4mm notochordal process and notochordal plate possessed a neural groove.
The neural folds on either side of the groove are only gentle elevations at this stage and almost entirely destined to form the future cephalic neural folds.
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.
The initial site of fusion of the neural folds is normally at the rhombencephalon part D or upper cervical part of the plate. It occurs when there are about 5 paired somites. The rostral extension of the closure at this stage is at rhombencephalon A but several sites may close independently, e.g. in Carnegie Embryo No. 5074. The maximum caudal extremity is at the level of the future 15/16th somites. Müller F., O'Rahilly R., (1985)
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 caudal neuropore is estimated to occupy 10% of the total length of the neural tube of embryos during this stage. The elongation and the closure of the neural tube progress at the same speed. Müller F., O'Rahilly R., (1986)
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 rostrally 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. Bartelmez G.W., Evans H.M., (1926) p58.
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) pp. 15. Presumably this is due to the lack of histological evidence and identification of a telencephalon specific structure
By the 25 somite stage the caudal neuropore is closed or closing and is invariably closed by the 27 somitic pair stage. The final level of closure is at about the level of the future 31st somite. Müller F., O'Rahilly R., (1987)