THE BRAIN OF A F@TAL ORNITHORHYNCHUS. 205 
Jacobson’s organ. n.olf. Olfactory nerves. opt.x. Optic nerve. 2.3, 
Oculo-motor nerve. G.g. Gasserian ganglion. 
Fie. 2.—Scheme of median sagittal section of the fore-brain, the hemi- 
sphere indicated by dotted lines. 6.0. Bulbus olfactorit. p.a. The “ pre- 
commissural area of the median cortex.” fdp. The rudiment of the hippo- 
campus (including the fascia dentata). p.c. Posterior commissure.  e.p. 
Epiphysis cerebri (pineal). s.c. Superior commissure. vel, Velum. ch.3. 
 Diaplexus”’ of Wilder. par. “ Paraphysis” of Selenka. 2 J/, Foramen of 
Monro. a.c. Commissura anterior, J/. ézf. Lamina infra-neuroporica., 7. 0. 
Recessus opticus of His (preeopticus—Burckhardt). opé.z. Optic nerve. 7.2. 
Recessus infundibuli of His (r. postopticus—Burckhardt). Ayp. Hypophysis 
cerebri. pit. Pituitary gland. 
Fic. 3.—Part of Fig. 2 enlarged.—/. sup. Lamina supra-neuroporica of 
Burekhardt. 1.2. Recessus neuroporicus (angulus terminalis, lobus olfac- 
toriusimpar). fc. Fornix commissure. 
Fig. 4.—Scheme of horizontal section through a mammalian brain, to show 
the relation of the ‘‘ precommissural area” (p.a.) to the lamina terminalis 
(2.¢.) and olfactory bulb (4.0.). Olf.y. Olfactory ganglion. c.s. Corpus 
striatum. ¢4. Diencephalon (His). 
Fie. 5.—Scheme of hypothetical arrangement of the terminal plate of the 
brain-tube, to show that the precommissural area and lamina terminalis are 
parts of one structure. Compare with Fig. 4. 
Fies. 6, 7, 9, 10, 11, 12, and 13 are a series of coronal sections of the 
fetal Ornithorhynechus brain, each x 12. The hippocampal rudiment 
(Aip.) may be seen extending through all the sections, lying in front above 
the precommissural area (p.a@.), which in turn is continuous with the tuber- 
culum olfactorium (¢.0.) and pyriform lobe (pyr.), on whose surface is the 
“external olfactory radiation” (Hdinger), e.0.7. yp. Pallium. c.e. External 
capsule. 
lig. 6.—Counting the sections from before backwards (each section being 
50 p thick), this is No. 24. It passes a short distance behind the olfactory 
bulb. 
Tig. 7.—(No. 35.) Just in front of the lamina terminalis. The falx cerebri 
(fx.) splits above to enclose the paraphysis (par.) and a number of large 
vessels. d./. Descending fornix fibres (Reichbundel). 
Fic. 8.—Section through the corresponding region in the adult brain. 
Weigert stain x 3. yp. Pallium. 4.f. Fissura hippocampi. f. d. Fascia 
dentata. d.f. Descending fornix fibres. p.a@. Precommissural area, ¢. 0. 
Tuberculum olfactorium. * Line of demarcation between p.a. aud Z. e. 
Jer. Fissura endorhinalis (Turner). pyr. Pyriform lobe. e. 0.7. External 
olfactory radiation. f.7. Fissura ectorhinalis. ¢.2. Internal capsule in 
corpus striatum (c.s.). ¢.7. Corona radiata. alv. Alveus. 
