FROM TWELVE TO THIRTY-SIX SOMITES 155 
and shows indication of branching. Subsequently it becomes 
much branched and quite massive and unites with the infun- 
dibulum to form the pituitary body. (See Chap. VIII.) 
The Mesencephalon. This portion of the brain comes to 
occupy the summit of the cranial flexure, which indeed owes its 
origin largely to the rapid growth in extent of the roof of the 
mesencephalon. In longitudinal section it thus appears wedge- 
shaped, with short floor and long arched roof (Figs. 87 and 
88). Its walls remain of practically uniform thickness up to 
the seventy-second hour. The lateral walls expand more rapidly 
than the roof and thus form the optic lobes. But these are 
barely indicated at the 36s stage. 
Isthmus. The great expansion of the mesencephalon does 
not involve the portion immediately adjacent to the hind-brain, 
which is henceforth known as the isthmus (Figs. 87, 88). 
The Rhombencephalon (Primary Hind-brain). Two divisions 
of the embryonic brain arise from the rhombencephalon, viz., 
the metencephalon and the myelencephalon; the former becomes 
the region of the cerebellum and pons of the adult brain, 
and the latter the medulla oblongata. The metencephalon is a 
relatively short section of the original rhombencephalon, and 
includes only the most anterior neuromere of the rhomben- 
cephalon or the sixth of the series (Fig. 83 D, KE). It may be 
distinguished at the beginning of the period under consideration 
by the fact that its roof remains as thick as that of the mesen- 
cephalon. At the end of this time, 7.e., seventy-two hours, the 
roof in sagittal sections appears to rise sharply from the isthmus 
and thins towards the summit, where it passes into the thin epi- 
thelial roof of the myelencephalon (Figs. 87 and 88). The rudi- 
ment of the cerebellum is slightly thicker on each side of the 
middle line at seventy-two hours. 
The myelencephalon becomes sharply characterized by the 
thinness of its roof and thickening of ventral lateral zones and 
floor. The epithelial roof has a triangular form, the base resting 
against the metencephalon. The neuromeres remain very distinct 
(Figs. 83, 89), but change their form. Up to about twenty-three 
somites they still form external expansions, but as the wall 
thickens the external surface becomes smooth, and the neuro- 
meres may now be recognized as a series of concavities in the 
lateral wall, with intervening projections (Fig. 89). The arrange- 
