| |
Chapter 15 5 Weeks: Cerebral Hemispheres
|
| |
| Between 4 and 5 weeks,
the brain continues
its rapid growth
and divides
into 5 distinct sections.
The head comprises about 1/3
of the embryo's total size.
The cerebral
hemispheres appear,
gradually becoming
the largest parts of the brain.
Functions eventually controlled
by the cerebral hemispheres
include thought, learning,
memory, speech, vision,
hearing, voluntary movement,
and problem-solving.
|
Chapter 16 Major Airways
|
| |
| In the respiratory system,
the right and left main stem
bronchi are present
and will eventually connect
the trachea, or windpipe,
with the lungs.
|
Chapter 17 Liver and Kidneys
|
| |
| Note the massive liver
filling the abdomen
adjacent to the beating heart.
The permanent
kidneys appear by 5 weeks.
|
Chapter 18 Yolk Sac and Germ Cells
|
| |
| The yolk sac contains
early reproductive cells
called germ cells.
By 5 weeks
these germ cells migrate
to the reproductive organs
adjacent to the kidneys.
|
Chapter 19 Hand Plates and Cartilage
|
| |
| Also by 5 weeks,
the embryo develops
hand plates,
and cartilage formation begins
by 5 1/2 weeks.
Here we see
the left hand plate
and wrist at 5 weeks and 6 days.
|
Embryonic Development: 6 to 8 Weeks
Chapter 20 6 Weeks: Motion and Sensation
|
| |
| By 6 weeks the cerebral
hemispheres are growing
disproportionately faster than
other sections of the brain.
The embryo begins
to make spontaneous
and reflexive movements.
Such movement is necessary
to promote normal
neuromuscular development.
|
| A touch to the mouth area
causes the embryo
to reflexively withdraw
its head.
|
Chapter 21 The External Ear and Blood Cell Formation
|
| |
| The external ear
is beginning to take shape.
|
| By 6 weeks,
blood cell formation
is underway in the liver
where lymphocytes
are now present.
This type of white blood cell
is a key part
of the developing immune system.
|
Chapter 22 The Diaphragm and Intestines
|
| The diaphragm,
the primary muscle
used in breathing,
is largely formed by 6 weeks.
|
| A portion of the intestine
now protrudes temporarily
into the umbilical cord.
This normal process,
called physiologic herniation,
makes room for other developing
organs in the abdomen.
|
Chapter 23 Hand Plates and Brainwaves
|
| |
| At 6 weeks the hand plates
develop a subtle flattening.
|
| Primitive brainwaves have been recorded
as early as 6 weeks and 2 days.
|
Chapter 24 Nipple Formation
|
| |
| Nipples appear
along the sides of the trunk
shortly before reaching
their final location
on the front of the chest.
|
Chapter 25 Limb Development
|
| |
| By 6 1/2 weeks,
the elbows are distinct,
the fingers are
beginning to separate,
and hand movement can be seen.
|
| Bone formation,
called ossification,
begins within the clavicle,
or collar bone,
and the bones
of the upper and lower jaw.
|
Chapter 26 7 Weeks: Hiccups and Startle Response
|
| |
| Hiccups have been observed
by 7 weeks.
Leg movements can now be seen,
along with a startle response.
|
Chapter 27 The Maturing Heart
|
| |
| The 4-chambered heart
is largely complete.
On average, the heart
now beats 167 times per minute.
Electrical activity of the heart
recorded at 7 1/2 weeks
reveals a wave pattern
similar to the adult's.
|
Chapter 28 Ovaries and Eyes
|
| |
| In females, the ovaries
are identifiable by 7 weeks.
|
| By 7 1/2 weeks,
the pigmented retina of the eye
is easily seen
and the eyelids
are beginning a period
of rapid growth.
|
Chapter 29 Fingers and Toes
|
| |
| Fingers are separate
and toes are joined
only at the bases.
|
| The hands can
now come together,
as can the feet.
Knee joints are also present.
|
The 8-Week Embryo
Chapter 30 8 Weeks: Brain Development
|
| |
| At 8 weeks the brain
is highly complex
and constitutes almost half of
the embryo's total body weight.
Growth continues
at an extraordinary rate.
|
Chapter 31 Right- and Left-Handedness
|
| |
| By 8 weeks, 75% of embryos
exhibit right-hand dominance.
The remainder
is equally divided
between left-handed dominance
and no preference.
This is the earliest evidence of
right- or left-handed behavior.
|
Chapter 32 Rolling Over
|
| |
| Pediatric textbooks describe
the ability to "roll over"
as appearing
10 to 20 weeks after birth.
However, this impressive
coordination
is displayed much earlier
in the low-gravity environment
of the fluid-filled
amniotic sac.
Only the lack
of strength required
to overcome the higher
gravitational force
outside the uterus prevents
newborns from rolling over.
|
| The embryo is becoming
more physically active
during this time.
Motions may be slow or rapid,
single or repetitive,
spontaneous or reflexive.
Head rotation, neck extension,
and hand-to-face contact
occur more often.
|
| Touching the embryo
elicits squinting,
jaw movement,
grasping motions,
and toe pointing.
|
Chapter 33 Eyelid Fusion
|
| |
| Between 7 and 8 weeks,
the upper and lower eyelids
rapidly grow over the eyes
and partially fuse together.
|
Chapter 34 "Breathing" Motion and Urination
|
| |
| Although there is no air
in the uterus,
the embryo displays intermittent
breathing motions by 8 weeks.
|
| By this time,
kidneys produce urine
which is released
into the amniotic fluid.
In male embryos,
the developing testes
begin to produce
and release testosterone.
|
Chapter 35 The Limbs and Skin
|
| |
| The bones, joints,
muscles, nerves,
and blood vessels
of the limbs
closely resemble
those in adults.
By 8 weeks the epidermis,
or outer skin,
becomes a multi-layered
membrane,
losing much of its transparency.
Eyebrows grow as hair
appears around the mouth.
|
Chapter 36 Summary of the First 8 Weeks
|
| |
| Eight weeks marks the end
of the embryonic period.
During this time,
the human embryo
has grown from a single cell
into the nearly
1 billion cells
which form over 4,000
distinct anatomic structures.
The embryo now possesses
more than 90% of the structures
found in adults.
|