In the fourth week the embryo grows from about 2 mm to about 5 mm in length (Fig. 32). The processes of the third week, the formation of the neural tube and of the blood circulation, continue. At the same time the amnion increases enormously in size, while the yolk sac does not. This difference causes a great change in form.
The growth of the amnion
In the third week the embryonic disc gained thickness and an axis of symmetry. The tissues of the embryonic disc continue into the tissues of the surrounding organs: the ectoderm into the membrane of the amnion, the entoderm into the wall of the yolk sac, and the mesoderm into the extra-embryonic mesoderm and the connective stalk. The embryo is still a disc that is open on all sides, with ectoderm on the back and entoderm on the front.
For an independent body to come into being, a skin has to form around the entire embryo. This happens through a dramatic growth of the amnion, which is pushed outward by the pressure of the amniotic fluid. The yolk sac does not grow and its fluid exerts no pressure, so the yolk sac hangs loose. Through these growth movements the amnion folds around the embryonic disc. This happens all around: at the head, at the tail and on both lateral sides, and faster at the head end than at the tail end.
Figures 28 to 31 show this enfolding. The three-dimensional process is difficult to imagine. Comparing the drawn sections with each other, and sketching further cross-sections, for example through the heart, makes it much easier to follow.
What happens?
- With the growth of the amnion the relatively large heart is moved from its cranial position in front of the head to its destination in the chest (Fig. 28 to 30).
- A little later the connective stalk is pushed towards the belly, to a place somewhat caudal of the heart. The umbilical cord will develop around it.
- The neural tube thickens quickly at the cranial end, which makes the head end fold forward and downward.
- The amnion pushes the yolk sac inward at the cranial, caudal and lateral sides. The firmness that the notochord gives the embryonic disc keeps it more or less straight. At the mouth membrane and at the cloacal membrane ectoderm and entoderm are joined, and these places act as hinges. This leads to the formation of the digestive tract (Fig. 29, 30).
- The formation of the digestive tract proceeds from the ends towards the centre.
- The umbilical cord is created in the centre. It attaches the embryo to the chorion and transports nutrients and waste products.
- The umbilical cord contains the connective stalk, the stalk of the yolk sac and the allantois (Fig. 31).
- When the amnion encloses the embryonic disc, part of the chorionic cavity is included in the embryo as the body cavity (the intra-embryonic coelom; Fig. 29 and 30).
- The embryo becomes more or less cylindrical, and then curves in its longitudinal direction (Fig. 31).
- Once it is formed, the digestive tract grows rapidly (Fig. 31).
Textbooks usually describe this enfolding as four folds that take place at the same time. The head fold brings the heart and the mouth membrane to the front of the body. The tail fold brings the connective stalk and the allantois to the belly side. The two lateral folds bend the sides of the disc downward until they meet in the midline and close the front wall of the body, except at the umbilicus. Together these folds turn a flat, three-layered disc into a tube within a tube: the gut tube inside, the body wall with the skin outside, and the body cavity between them.
The embryo at day 24 and day 28
Fig. 32 shows two side views of the embryo in the fourth week. In the drawing from the middle of the week, day 24 (Fig. 32 left, corresponding to Fig. 29), the amnion surrounds the embryonic disc and the cylindrical embryo has been created. The large bulge of the heart is visible; it has been pushed from its cranial position in front of the head to a place in the chest. The neural tube is still open at both ends. The connective stalk and the yolk sac are visible.
At the end of the fourth week, day 28 (Fig. 32 right, corresponding to Fig. 31), the amnion encloses the embryo completely, and the umbilical cord with its blood vessels, connective stalk, yolk sac and allantois is visible. The heart has grown, has descended a little further and now lies in its final region, just above the umbilical cord. The embryo has a short tail and has become rounder. Head and tail are bent inward.
The head is large, because the head end develops faster than the tail end. Folds have appeared at the head, caused by the fast thickening of the tissue of the neural tube, which bends the head downward. The first fold is the mandibular arch, from which the upper and lower jaw develop. The folds behind it are called pharyngeal or branchial (gill) arches, because they resemble the arches that carry the gills in fish. In humans they give rise to parts of the face, the ear and the neck, and no gills develop. The first beginnings of an eye and an ear are visible. The limb buds appear as well, first those of the arms and shortly after those of the legs.
Neural tube and digestive tract
The neural tube arises from the centre, the digestive tract from the ends. In the neural tube there is clear fluid, which comes from the amniotic fluid and exerts pressure on the wall. In the digestive tract there is turbid yolk fluid, which does not. The neural tube is mainly a straight tube; the digestive tract is a tube that winds in and out, as the intestines do. The neural tube consists of ectoderm, the digestive tract is lined with entoderm.
The phenomenological approach reads more into these contrasts. Ectoderm comes from the back, which may be called the antipathy side of the body, as in the expression to turn one's back on someone. Entoderm comes from the front, the sympathy side of the body. With the ectoderm, the skin and the senses, a person sets boundaries and creates distance, which is the condition for observing. With the entoderm a person makes connections, as in the digestion of food, where something from the outside is taken in and made one's own.
| neural tube | digestive tract | |
|---|---|---|
| arises from | the middle | the ends |
| fluid | clear amniotic fluid, exerts pressure | turbid yolk fluid, no pressure |
| tissue | ectoderm | entoderm |
| form | straight | winding |
| where | back = antipathy side | front = sympathy side |
Characteristics
The processes of the third week gave the embryo volume, but did not give the embryonic disc a boundary. At the beginning of the fourth week the embryonic disc still continues into the enclosing tissues of amnion, chorion and yolk sac. The enfolding movement of the amnion separates the embryo from these tissues and makes the embryonic disc independent. It still takes a long time before the embryo can live on its own, but the beginning is there: the body gets its first form and is connected to the nourishing tissues only by the umbilical cord. Rudolf Steiner called this stage "Paradise Man", meaning the first separation of the human being from the environment.
Now the body stands apart and is three-dimensional. It bends and is directed towards a centre (Fig. 32). Hartmann uses the name "Animal Man" for this stage (Fig. 33), in contrast to the "plant-man" of the second week. The comparison between the two gestures is worked out on the page plant and animal.
The enfolding movement of the amnion has the result that the back now lies on the outside, while what was on the ventral side now lies inside. It is an enveloping gesture.
The umbilical cord lies on the ventral side of the embryo. It began as the connective stalk at the back, then moved to the tail, and now it has shifted to the abdomen. In the stage of the plant-man the nourishment came from behind; in the stage of the animal man it comes from the front.
The three germ layers and what arises from them
The embryonic disc now consists of three germ layers: ectoderm, entoderm and mesoderm. Each gives rise to its own group of tissues and organs.
From the ectoderm arise:
- the skin (its outer layer, the epidermis, with hair and nails)
- the nervous system
- the senses
From the entoderm arise:
- the lining of the digestive tract and the digestive glands (liver, pancreas)
- the lungs
- the bladder, whose development is linked to the allantois and the end of the gut
The mesoderm can form three kinds of tissue:
- It can concentrate and grow inward to form muscles, tendons, ligaments and bones, and also the kidneys, the spleen and the reproductive organs.
- It can move to the periphery and line the body cavities, such as the pericardium, the pleural cavity and the abdominal cavity.
- It can do both at once and form blood cells, blood vessels and the heart.
This third, mediating capacity of the mesoderm is where the circulation came from in the third week, and it is the heart, formed in the flow of the blood, that the folding of the fourth week has now brought to its place in the chest.
Questions
What happens to the human embryo in the fourth week?
The flat embryonic disc folds into a curved, cylindrical body. The amnion grows around it on all sides, the heart moves to the chest, the gut tube and the umbilical cord form, and the first arches of the head, the beginnings of eye and ear and the limb buds appear.
Why does the embryo fold?
The amnion and the embryonic disc grow fast while the yolk sac does not. The pressure of the amniotic fluid pushes the amnion outward, and the faster-growing tissues fold the edges of the disc downward and inward, at the head, the tail and both sides.
How is the digestive tract formed?
When the edges of the embryonic disc fold inward, part of the entoderm-lined yolk sac is taken into the body and closes into a tube. This happens from the head end and the tail end towards the middle, where the connection with the yolk sac remains for a time in the umbilical cord.
How does the heart reach the chest?
At the start of the fourth week the heart lies in front of the head end of the embryonic disc. The folding of the head end swings it downward and inward, so that it comes to lie in the future chest, below the mouth.
What is in the early umbilical cord?
It contains the connective stalk with the umbilical blood vessels, the stalk of the yolk sac and the allantois, all covered by the amnion. It connects the embryo with the chorion, through which nutrients and waste products are exchanged.
What is the intra-embryonic coelom?
It is the part of the cavity system that is enclosed within the body when the embryo folds. From it the body cavities develop: the pericardial cavity around the heart, the pleural cavities around the lungs and the abdominal cavity.
What are the pharyngeal or branchial arches?
They are ridges on the side of the head and neck region of the embryo at the end of the fourth week. The first arch gives rise to the upper and lower jaw; the others contribute to structures of the face, ear and neck. They resemble the gill arches of fish embryos, but in humans no gills develop.
Which organs come from the three germ layers?
The ectoderm gives rise to the skin, the nervous system and the senses. The entoderm gives rise to the lining of the digestive tract, the liver, the pancreas and the lungs. The mesoderm forms muscles, bones, connective tissue, kidneys, the heart, blood and blood vessels and the linings of the body cavities.
Why is this stage called Animal Man?
In the phenomenological reading the embryo now has a body with content that is curved around a centre, the gesture of an animal. The name comes from Hartmann; in the same tradition Rudolf Steiner spoke of Paradise Man for this first separation of the human being from the environment.
How big is the embryo at the end of the fourth week?
It grows from about 2 mm to about 5 mm in length during the fourth week. At that size the head is already large compared with the rest of the body, because the head end develops fastest.