(formation of the body flexion, allantois and placenta)
The next stage of the embryonic development, which happens on about the 20th day, is one of the most important parts of development and includes the separation of the embryo body from the extraembryonic structures. This process begins at the same time with the formation of the axial organs complex.
It must be studied on the transverse and longitudinal sections. Of the first, the embryo rises up into the amniotic cavity. At that time the amniotic cavity, filled up with the amniotic liquid, enlarges and goes down, covering the embryo body and yolk sac, which is empty and may be pressed with ease.
So, the amnion increases, while the yolk sac decreases in measurement. This amnion enlargement leads to the formation of the head and tail folds of the embryo. Further the embryo plate elongates, flexes and approximates its cephalic and caudal extremities ventrally.
With the formation of the head and tail folds, part of yolk sac becomes enclosed within the forming embryo body and a tube, lined by the endoderm, is formed. This is the primitive gut, from which the biggest part of the gastrointestinal tract is formed. The communication with the yolk sac becomes narrower and narrower and eventually disappears.
In the mesoderm of the flattened yolk sac the first blood vessels appear- this is the function of the human embryo yolk sac.
From the caudal end of the primitive gut an empty tube grows into the connecting stalk – allantois, the significance of which in the human is the formation of the urinary bladder and blood vessels, growing to the forming placenta (see chapter Female reproductive organs).
Enlarged cavity of the amnion presses the yolk sac and allantois, that form the umbilical cord.
Late embryonic stages
Histogenesis and organogenesis
At the late embryonic stages the differentiation of tissues and organs of the embryo body begins. These changes are very important and will be studied during the learning of the Histology course.
Differentiation of the germ layers
Germ layers
(embryonic sources)
| Ectoderm | Endoderm | Mesoderm |
| (external) | (internal) | (middle) |
Ectoderm
| Skin ectoderm | Neuroectoderm | Extraembryonic ectoderm |
Skin ectoderm
| Epidermis and its derivatives | Epithelium of oral mucosa and anal part of a rectum | Tooth enamel |
| (glands of skin, hairs, nails) | and their glands |
