Vt

Ventral thalamus

vz

Ventricular zone

wm

White matter

zl

Zona limitans

alar rostral diencephalon consists of three prosomeric subregions labeled caudal (p4), intermediate (p5), and rostral (p6) prethalamic areas (Fig. 4). The optic vesicles evaginate out of the rostral area at early neurulation stages (Figs. 1A-1F). Part of the pretha-lamus, particularly the rostral part, has been attributed to the hypothalamus, but it is convenient to restrict the term "hypothalamus" to the basal part of the rostral diencephalon. This includes mammillary/subthalamic (p4), tuberomammillary (p5), and tuberal hypophyseal regions (p6) (Fig. 4). In this way, the rostral dience-phalon divides into optic vesicles and prethalamus (alar plate) and hypothalamus (basal and floor plates).

ones of Herrick and Kuhlenbeck. The primary pro-sencephalon, or early forebrain vesicle, soon divides into two principal transverse subdivisions: caudally, the caudal diencephalon, and rostrally, the secondary prosencephalon (sum of rostral diencephalon and telencephalon; Figs. 1D-1F, 2). In our description, the conventional hypothalamus is constituted exclusively by parts of the rostral diencephalon and is independent from the caudal diencephalon.

The caudal diencephalon abuts caudally the mid-brain; it has three transverse subdivisions, postulated to be prosencephalic neuromeres (prosomeres p1-p3) (Figs. 2 and 3). These contain the pretectal (p1), dorsal thalamic/epithalamic (p2), and ventral thalamic (p3) regions. Each prosomere has alar (dorsal) and basal (ventral) components; the aforementioned three large regions are the respective alar components of p1-p3 (Fig. 3). The basal components jointly form the prerubral tegmentum.

The transverse organization of the secondary prosencephalon (SP) is more hypothetical, but it has been postulated to contain three prosomeres as well (p4-p6), which are readily distinguishable in the rostral diencephalon, though it is not clear how they relate to the overlying telencephalon (Fig. 2). The telencepha-lon is a dorsal evaginated region within the SP (T in Figs. 1D-1F), and lies strictly dorsal to the rostral diencephalon. The latter also appears divided into alar and basal plates, like its caudal counterpart. The alar plate extends from the telencephalon down to approximately the ventral limit of the optic tract. Thus, the optic stalk and the optic tract lie in a longitudinal alar plate domain that traverses the rostral diencepha-lon, thalamus, pretectum, and midbrain (Fig. 2). The

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