In Mammalia the cerebellum attains a still greater development. The median lobe or vermiform process is first developed. In the higher Mammalia the lateral parts forming the hemispheres of the cerebellum become formed as swellings at the sides at a considerably later period, and are hardly developed in the Monotremata and Marsupialia.
Fig. 250. Longitudinal section through the brain of a Chick of ten days. (After Mihalkovics.)
hms. cerebral hemispheres; alf. olfactory lobe; alf1. olfactory nerve; ggt. corpus striatum; oma. anterior commissure; chd3. choroid plexus of the third ventricle; pin. pineal gland; cmp. posterior commissure; trm. lamina terminalis; chm. optic chiasma; inf. infundibulum; hph. pituitary body; bgm. commissure of Sylvius (roof of iter a tertio ad quartum ventriculum); vma. velum medullæ anterius (valve of Vieussens); cbl. cerebellum; chd4. choroid plexus of the fourth ventricle; obl4. roof of fourth ventricle; obl. medulla oblongata; pns. commissural part of medulla; inv. sheath of brain; bls. basilar artery; crts. internal carotid.
The cerebellum is connected with the roof of the mid-brain in front and with the choroid plexus of the fourth ventricle behind by delicate membranous structures, known as the velum medullæ anterius (valve of Vieussens) and the velum medullæ posterius.
The pons Varolii is formed on the ventral side of the floor of the cerebellar region as a bundle of transverse fibres at about the same time as the olivary bodies.
The mid-brain. The changes undergone by the mid-brain are simpler than those of any other part of the brain. We have already seen that the mid-brain, on the appearance of the cranial flexure, forms an unpaired vesicle with a vaulted roof and curved floor, at the front end of the long axis of the body ([fig. 118], MB). It is at this period in most Vertebrates relatively much larger than in the adult; and it is only in the Teleostei that it more or less retains in the adult its embryonic proportions.
The cavity of the mid-brain, greatly reduced in size in the higher forms, is known as the iter a tertio ad quartum ventriculum, or aqueductus Sylvii.
The roof of the mid-brain is sharply constricted off from the divisions of the brain in front of and behind it, but these constrictions do not extend to the floor.
In some Vertebrates the region of the mid-brain is stated to undergo hardly any further development. In the Axolotl it remains according to Stieda[163] as a simple tube with nearly uniformly thick walls. In the majority of forms it undergoes, however, a more complicated development.