84 MARION HINES 
nervous system. The basal plate of the cord has grown much 
thicker. The pontile flexure is evident, but not as accentuated 
as His pictured for the 10.6-mm. in his collection; nor is the 
floor plate of the medulla oblongata as he showed it. The lateral 
recess has appeared. There is no indication of a cerebellar 
thickening in the superior lip of that recess. The midbrain is 
not as prominent a feature of the morphology of this brain, due 
in part to the acceleration in growth of the diencephalon. The 
mesencephalon is separated from the rhombencephalon by a 
marked constriction of the total brain tube, the isthmus. In 
the 14-mm. embryo the transition from midbrain to the dien- 
cephalon is marked off distinctly in the midline by a sudden 
dip in the vault. The diencephalic portion of the invagination 
is the posterior limb of the epiphyseal evagination. The basal 
plate of the midbrain has grown toward the lumen of the 
ventricles. 
The diencephalon is divided into three parts, comparable to 
those described for the thalamic region of the 11.8-mm. embryo, 
by the sulcus limitans below and the dorsal sulcus above. The 
absolute distance between the dorsal sulcus (fig. 14, Sul. dors.) 
and the sulcus limitans (fig. 14, Sul. lim.) is greater here than in 
the younger embryo, but the tissue dorsal to this ridge and that 
ventral to the sulcus limitans has changed little. Above the 
dorsal sulcus is a well defined ridge which is still more clearly 
marked in the 19.1-mm. embryo (fig. 15). The diencephalic 
roof plate is longer, measuring the distance from the velum trans- 
versum (fig. 14, Vel. trans.) to the epiphyseal evagination (fig. 
14, Ep. ev.). The definitive regions of the floor are already 
outlined. The wall of the recessus mamillaris is not as shallow 
as that of the 11.8-mm. embryo. The recessus infundibuli is 
now a definite well in the floor, dipping down into the solid 
stalk of the posterior lobe of the hypophysis. The bed of the 
optic chiasma has increased in breadth and thickness. The 
preoptic and postoptic recesses are actual cavities in the hypo- 
thalamic floor. The sulcus limitans ends blindly dorso-caudal 
to the optic ventricle. 
