squamosals, exoccipitals, and pro-otics. The quadrato-jugals are very inconspicuous. The extrastapedial plate of cartilage is well marked and the bony columella may be seen lying underneath it. The sides of the skull are ossified and may represent the orbitosphenoids and alisphenoids of higher types. The supra-occipital consists of a piece of cartilage.

Fig. 15.—Dorsal view of Skull of Xenopus and Rana.

[In the Frog there is no supra-ethmoid present, but a sphenethmoid closes the front of the cranial cavity. The sides of the skull are cartilaginous. A quadrato-jugal is well marked, but the quadrate consists of a piece of cartilage at the posterior end of the quadrato-jugal between the pterygoid and the squamosal. The lower jaw articulates with it. The columella is usually not well shown.]

Make a drawing of the dorsal side of the skull.

2. Remove the skin from the roof of the mouth and note the parasphenoid and single vomer, pterygoids, which are large and send leaf-like processes towards the middle line covering the Eustachian passages, squamosals, quadrates, basi-occipital, ex-occipitals, premaxillæ, and maxillæ. The orbitosphenoid and alisphenoid are well marked. There are no palatines present.

Fig. 16.—Ventral view of Skull of Xenopus and Rana.

[In the Frog there are a pair of vomers bearing the vomerine teeth, palatines, and quadrato-jugals. The pro-otics, which are not seen in a ventral view of the skull in the Platana, being covered over by the pterygoids, are shown in the Frog.]

Make a drawing of the ventral side of the skull.

Fig. 17.—Side view of Skull of Xenopus and Rana.

3. Make a drawing of the bones of the skull viewed from the posterior end, showing the occipital condyle at each side of the foramen magnum, ex-occipitals, basi-occipital, supra-occipital, pro-otics, columella, and quadrate.

[In the Frog the basi-occipital and supra-occipital are not developed.]

4. Remove the skin from the lower jaw and make out the bones surrounding the original Meckel's cartilage, viz., the mento-meckelian, dentary, and articular.

[In the Frog the lower jaw is composed of the mento-meckelian, dentary and angulo-splenial.]

Make a drawing of the lower jaw.

Nervous System.

A. The Central Nervous System.

The Brain.

Carefully remove the fronto-parietals so as to expose the brain, and note the following parts:—

(a) The olfactory lobes, forming the most anterior portion of the brain, united together in the middle line. Anteriorly the olfactory nerves are given off.

(b) The cerebral hemispheres, a pair of smooth elongated bodies.

(c) The thalamencephalon, lying immediately behind the cerebral hemispheres. It is covered by the choroid plexus, a thick vascular membrane, and gives rise dorsally to the stalk of the pineal body.

(d) The optic lobes, a pair of oval bodies.

(e) The cerebellum, a narrow transverse band immediately behind the optic lobes.

(f) The medulla oblongata behind the cerebellum. It gradually tapers towards its posterior end, where it is continuous with the spinal cord. On its roof the pia mater forms a very vascular choroid plexus.

Make a sketch of the brain.

Seventh Day. (Revision.)

With a second specimen revise the second, third, and fourth days' work, paying particular attention to the vascular system.

Eighth Day.

Nervous System (continued).

B. The Peripheral Nervous System.

1. The Spinal Nerves.

Remove the abdominal viscera carefully so as to expose the spinal nerves which are seen as white cords on each side of the vertebral column as follows:—

(a) The first or hypoglossal nerve arising between the 1st and 2nd vertebræ. It runs forwards on the under surface of the head beneath the mylo-hyoid muscle and supplies the tongue. It was noted in the third day's work.

(b) The second and third nerves unite immediately after escaping from the spinal cord to form the brachial plexus, which supplies the muscles of the shoulder, and then extends along the arm.

(c) The fourth, fifth, and sixth nerves are small and supply the muscles and the skin of the body wall.

(d) The seventh, eighth, and ninth nerves run obliquely backwards, the eighth and ninth uniting to form the sciatic plexus, which gives rise to the large sciatic nerve which extends along the thigh, and down the leg. The sciatic plexus also receives a branch from the seventh nerve.

(e) The tenth or coccygeal nerve, which escapes through a small aperture in the side of the urostyle, and is distributed to the walls of the cloaca and urinary bladder. It is connected by a branch with the sciatic nerve. This nerve is frequently small or absent in the Platana.

[In the Frog, the seventh, eighth, and ninth nerves unite to form the sciatic plexus which gives rise to the large sciatic nerve extending along the thigh, and dividing above the knee into the tibial and peroneal nerves to the leg and the foot. The tenth nerve is present.]

2. The Sympathetic Nervous System.

This system consists of a cord on each side closely following the course of the systemic arches and the dorsal aorta. It has on it six ganglionic swellings, and there are connections between the ganglia and all the spinal nerves. Anteriorly each of the cords is joined to the vagus nerve; it ends posteriorly as a broad swelling on the ninth spinal nerve.

[In the Frog the system consists of a chain of ganglia on each side of the body closely following the course of the systemic arches and the dorsal aorta. The cords have connections with the spinal nerves. Anteriorly each cord ends in the Gasserian ganglion of the trigeminal nerve and posteriorly it is united with the coccygeal nerve by a varying number of branches.]

Make a complete sketch showing the spinal nerves and the sympathetic nervous system.

3. The Cranial Nerves.

Expose the brain and note the following cranial nerves:—

(a) The olfactory nerve, arising from the anterior end of the olfactory lobes, supplying the olfactory capsule.

(b) The optic nerve, arising from the ventral surface of the brain below the optic lobes, supplying the retina of the eye.

(c) The