What Lies Beyond The Gastrula

From our discussion, we can say that the gastrula is basically a three-layered embryo. The gastrula stage usually occurs during the second week after fertilization. Figure 15.7 displays some of the developments that occur thereafter.

A cylinder-shaped body appears

As the developing cell layers in the gastrula divide and widely migrate, the three germ layers roll up into a hollow cylinder, which becomes the General

Head end

Somites

Neural tube

Eye begins Ear begins

Head end

Eye begins Ear begins

Somites

Neural tube

Somites Limb Buds

Limb buds appear

Paddle-shaped hands & feet

Toes separated

56 days

Tail

Somites'

Limb buds appear

Paddle-shaped hands & feet

Toes separated

Somites'

23

24

28

40

days

days

days

days

Fig. 15.7 The embryo develops beyond the gastrula.

56 days

Fig. 15.7 The embryo develops beyond the gastrula.

Cylinder-shaped Body Plan. At this stage, in other words, we humans look much like a chubby little caterpillar - one with a stubby tail!

At just after 3 weeks (around the 23rd day), the cylinder-shaped body contains a neural tube in its back, flanked by rows of somites (developing body segments).

Membranes appear around the body

At about the same time as the General Cylinder-shaped Body is forming, a number of membranes appear around the Body. A glance back at Figure 15.6, for example, shows the amnion (AM-nee-un). The amnion is a protective membrane forming a sac around the embryos of ''little lambs'' (amnions), human beings, and other types of mammals. The amnion also encloses a quantity of amniotic (am-nee-AH-tik) fluid. This fluid creates a watery cushion and shock absorber for the embryo, as well as keeping the body wet and moist.

Another membrane becomes the yolk sac, which provides nourishment for the early embryo. By 24 days, a connecting stalk appears in the middle of the now worm-like body. The yolk sac hangs off to one side of this connecting stalk. Both attach to a primitive placenta (plah-SEN-tah), a ''flat cake'' (placent) of highly vascular (blood vessel-rich) tissue that nourishes the developing embryo and later, the fetus.

Organogenesis goes into full swing

During the same time that the membranes (such as the amnion) are forming around the outside of the embryo, organogenesis (or-gan-oh-JEN-eh-sis) -''production of'' (-genesis) living ''organs'' (organo-) - is proceeding at a fast pace inside of the embryo. By 24 days (32 weeks), for instance, a miniature heart organ has developed and is vigorously pumping blood for a body that is less than 1/4 of an inch long!

Limb buds sprout

By 28 days, upper and lower limb buds sprout out from the sides of the grublike body, which now seems to be getting some eyes! These buds mark the locations for later development of true body limbs.

An umbilical cord appears

By 40 days, an umbilical (um-BILL-ih-kal) cord has replaced the yolk sac and connecting stalk, and has firmly rooted itself into the nourishing placenta. Upper and lower appendages have grown out from the limb buds, and the hands and feet look like little paddles. A definite eye with pigment is now visible.

End of embryo stage: All major organ systems have developed

By 56 days (8 weeks or 2 months), the embryo body has a distinctly human form. The tail is long gone, and the fingers and toes are completely separated from one another in the hands and feet.

''Can my little brother see me, yet?'' Not quite yet, Baby Heinie. The eyes are present and are well-developed, but the eyelids are stuck together! ''You mean ... They're keeping my little brother in the dark?"

After about 56 days (2 months or so), organogenesis has been completed. All major body tissues and organ systems are up and functioning! The embryo stage is nearly over! (It officially ends at 9-12 weeks - the third month.)

Good! My Mom is really pregnant! Grandma says she's 8 weeks along! Can I see my little brother's face, Good Professor Joe?'' Okay, here he is!

  • Yikes! Why is my little brother's head so big? Is that normal gross anatomy, Prof?" At this stage of development, it is normal. At 8 weeks, the head is almost as big as the rest of the body! But, not to worry! The rest of the body will rapidly lengthen.
  • Well, how long is my little brother at 8 weeks, and how much does he weigh?' At the end of the embryo stage, the body is about 1.2 inches (or 30 mm) long, and it weighs about 2 grams (0.06 ounce).
  • Well, maybe my little brother can't see, but at least he won't be born with a weight problem!''
0 0

Post a comment