Prosencephalon Telencephalon Cerebral cortex,
(forebrain) corpus striatum
Diencephalon Hypothalamus, thalamus
Mesencephalon Mesencephalon Superior and inferior
(midbrain) colliculi
Rhombencephalon Metencephalon Pons and cerebellum
(hindbrain)
Myelencephalon Medullawall of the developing CNS contains three layers: ventricular, mantle
(intermediate), and marginal zones. The cortex, peripheral areas of gray
matter, is formed through the migration of cells from the mantle zone to
the marginal zone. Segmentation of the cranial neural tube forms the brain
vesicles listed in the table below.
92 Anatomy, Histology, and Cell Biology
Primary Brain Secondary Brain Adult Brain
Vesicle Vesicle Derivative25.The answer is c.(Sadler, pp 11–12 .Moore and Persaud, Developing,
pp 143, 304, 306, 308, 326.)The primordial germ cells are first seen in the
endodermal lining of the wall of the yolk sac (derived from the hypoblast)
at the end of the third or beginning of the fourth week in the region of the
allantois. During embryonic folding, the dorsal part of the yolk sac is incor-
porated into the embryo as the primitive gut. The primordial germ cells
subsequently migrate along the dorsal mesentery of the hindgut (answer a)
and into the gonadal (genital) ridge by week 6 (answer b).The primary
sex cords grow into the mesenchyme underlying the ridge, and the pri-
mordial germ cells become incorporated into the primary sex cords
(answer d).The chorion (answer e)is the outermost fetal membrane and
is composed of extraembryonic somatic mesoderm, cytotrophoblast, and
the syncytiotrophoblast. It is divided into the chorion frondosum, where
the villi form and proliferate, and the smooth chorion, also known as the
chorion laevae. The following diagram illustrates the arrangement of the
fetal membranes.