(mess- = “middle”) is the next vesicle, which
can be called the midbrain. The third vesicle at this stage is the rhomb
encephalon. The first part of this word is also the root of the word rhombus,
which is a geometrical figure with four sides of equal length (a square is a
rhombus with 90° angles). Whereas pros encephalon and me encephalon translate
into the English words forebrain and midbrain, there is not a word for
“four-sided-figure-brain.” However, the third vesicle can be called the
hindbrain. One way of thinking about how the brain is arranged is to use these
three regions—forebrain, midbrain, and hindbrain—which are based on the primary
vesicle stage of development ([link] a).The brain continues to develop, and the
vesicles differentiate further (see [link] b). The three primary vesicles
become five secondary vesicles. The pros encephalon enlarges into two new
vesicles called the epencephalon and the diencephalon. The telecephalon will
become the cerebrum. The diencephalon gives rise to several adult structures;
two that will be important are the thalamus and the hypothalamus. In the
embryonic diencephalon, a structure known as the eye cup develops, which will
eventually become the retina, the nervous tissue of the eye called the retina.
This is a rare example of nervous tissue developing as part of the CNS
structures in the embryo, but becoming a peripheral structure in the fully
formed nervous system.The me encephalon does not differentiate into any finer
divisions. The midbrain is an established region of the brain at the primary
vesicle stage of development and remains that way. The rest of the brain develops
around it and constitutes a large percentage of the mass of the brain. Dividing
the brain into forebrain, midbrain, and hindbrain is useful in considering its
developmental pattern, but the midbrain is a small proportion of the entire
brain, relatively speaking.The rhomb encephalon develops into the met
encephalon and myelencephalon. The met encephalon corresponds to the adult
structure known as the puns and also gives rise to the cerebellum. The
cerebellum (from the Latin meaning “littl
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