X. THE PRIMITIVE VERTEBRATE BRAIN 
The dogfish, a small species of shark, represents one of the 
most primitive groups of fishes, and its brain shows the simple 
form and arrangement of parts found in early embryos of higher 
vertebrates. It thus furnishes the key to the interpretation of 
brain structure in higher forms such as the mammals. 
A. DORSAL ASPECT OF THE DOGFISH BRAIN IN SITU. 
First, remove the skin, then carefully slice away the cartilage 
which forms the brain case (the animal has no bones) until the 
entire brain is exposed. The cartilage is translucent, and, if the 
dissection is done with care, before the nerves are reached they 
may be seen passing through the foramina and they may thus be 
preserved in connection with the brain. 
Study and draw the brain from the dorsal side , in situ, identifying 
the following brain vesicles and their parts: 
1. Telencephalon. —Olfactory lobes, cerebral “hemispheres.” 
2. Diencephalon. —Optic thalami, epiphysis (pineal body). 
3. Mesencephalon. —Optic lobes. 
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4. Metencephalon. —Cerebellum, with a median and two 
lateral lobes (flocculi). 
5. Myelencephalon. —Medulla oblongata. 
B. MEDIAN SAGITTAL SECTION OF THE DOGFISH 
BRAIN IN SITU. 
This section may be made by a single stroke by means of the 
w T et blade of a sharp thin knife or a section cutter through the 
median plane of the entire head. Study the brain as it appears 
in median sagittal section, identifying the following features, 
and recording the facts learned by means of a suitable drawing. 
1. Telencephalon. —Note the thickening, the corpus striatum, 
in the floor on each side, and the thinness of cerebral roof, or 
pallium, which forms the cerebral hemispheres. Each cerebral 
hemisphere contains a ventricle, the first and second, and these 
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