THE FISSURA HIPPOCAMPI 83 
evagination (Rec. op.). Immediately dorsal to this groove in 
the diencephalon lies another, here termed sulcus dorsalis (fig. 
8, Sul. dors.), which to all appearances arises from the sulcus 
limitans rostral to the meso-diencephalic boundary. Further 
forward the sulcus limitans and the sulcus dorsalis fade out in the 
thalamic wall, dorsal and anterior to the optic ventricle. 
These two sulci divide the diencephalon into three regions: 
a large dorsal region, an insignificant central region, here termed 
the midthalamic region, and a large ventral region, the hypo- 
thalamus. ‘The first or epithalamus is bounded rostrally by the 
velum transversum (fig. 8, Vel. trans.), dorsally by a thin ependy- 
mal roof (fig. 8, Dien. r. pl.), and the epiphyseal evagination 
(fig. 8, Hp. ev.). The floor of the hypothalamus contains the 
shallow mammillary recess (fig. 8, Corp. mam.), the broad in- 
fundibular area (fig. 8, Znf.), and the bed of the optic chiasma 
(fig. 8, F. b. op. ch.). The infundibulum can be identified as that 
area to which the anlage of the anterior lobe of the hypophysis 
clings (fig. 8, Ané. l. hyp.). 
The midline of the telencephalic roof is only a little lower 
than the two lateral vaults of the hemispheres. Consequently 
the foramen interventriculare (fig. 8, For. it.) is only a little 
smaller than the entire cavity of the whole evagination. ‘The 
only point of constriction lies in the most dorsal part of the di- 
telencephalic groove, the region of the velum transversum (fig. 
8, Vel. trans.). 
Following the midline structure forward from the velum 
transversum, it remains membranous as far as the massive lam- 
ina terminalis (fig. 8, Lam. term.), below which a slight con- 
striction marks the preoptic recess (fig. 8, Rec. preop.). Next 
comes the chiasma ridge (fig. 8, F. b. op. ch.) at the di-telence- 
phalic junction. The corpus striatum is barely visible as a 
slight ventricular eminence in the floor of the interventricular 
foramen. 
The 14-mm. embryo, Mall Collection, 940 (figs. 10, 12, 15, 14) 
A difference of only 2.2 mm. between this embryo and no. 
1121 has wrought a marked change in the growth of the central 
