The third week: the embryo gets direction

In the second week the embryo grew explosively in the periphery. The tissue between the outer trophoblast and the inner embryoblast tore apart, and the chorionic cavity arose. If this pattern of strong growth in the periphery and little growth inside continued, a growing and ultimately unbridgeable distance would separate the nutritive trophoblast from the embryonic disc. In the third week a reversal takes place. The periphery keeps developing, but the focus of development now lies on the embryonic disc.

Several developments occur more or less simultaneously. For easy reference they are discussed separately here, in the order in which they begin.

Gastrulation: the formation of mesoderm tissue

The first process of the third week is called gastrulation (gaster = stomach). In simple animals gastrulation means the formation of a stomach and a digestive tract, as described on the page plant and animal, and at the same time the body cavity in which the mesoderm develops. In humans, gastrulation is the process by which the mesoderm is formed and the third germ layer comes into being. The digestive tract develops later, in a process in which the amnion is involved.

At the beginning of the third week the tissue of the epiblast thickens and a small groove forms in the embryonic disc near the connective stalk. This happens at the caudal end of the disc (cauda = tail, caudal = at the tail end; Fig. 20 and 21A). This formation is called the primitive streak. It grows in cranial direction, towards the side of the head, away from the connective stalk. Epiblast cells move through the primitive streak and migrate into the space between the epiblast and the hypoblast. Some of these newly formed cells also replace the cells of the hypoblast.

The primitive streak grows to the middle of the embryo (Fig. 21B). There it forms the primitive knot, the place where most mesoderm cells arise. These cells migrate to every place between the epiblast and the hypoblast, except for the prochordal plate and the places where the mouth and anus will arise, because the two layers stick together there. From now on the epiblast is called ectoderm and the lower layer entoderm.

Then a tube grows from the primitive knot between the two layers to the prochordal plate. It later becomes a solid rod, the notochord (chorda dorsalis; Fig. 21C). This flexible rod gives the embryonic disc firmness and creates an axis of symmetry in the embryo. With the notochord, left and right are defined; the mesoderm gives the embryonic disc content and thickness. The vertebral column will later form around the notochord, and in adults small remnants of it remain in the intervertebral discs.

At the same time the embryo grows rapidly in length and much less in width, and faster at the cranial side than at the caudal side. Its form therefore changes from round to oval, later to pear-shaped. Because growth is fastest at the cranial end, the primitive streak becomes relatively smaller and eventually disappears.

The neural tube and the somites

The genesis of the neural tube is linked to the growth of the notochord. From about the 19th day the ectoderm above the notochord becomes thicker (Fig. 22). This thickened area, about half of the ectoderm, is called the neural plate. The central nervous system will develop from it. The rest of the ectoderm does not thicken and will become the outer layer of the skin.

The embryonic disc grows fast: in five days it doubles its length. On the cranial side it grows much faster than on the caudal side, which makes the neural plate at the head end longer and wider than at the tail end. The thick neural plate (Fig. 23 and 24) folds inward, first in the middle and from there towards the cranial and caudal ends. The fold deepens into the neural groove. Then the edges of the groove meet, the connection with the surrounding ectoderm is broken and the ectoderm closes over it. The neural tube has formed.

Closure starts in the middle of the embryonic disc and proceeds towards both ends. It is completed in the fourth week: the opening at the head end closes around day 25, the opening at the tail end a few days later, around day 28.

The neural tube encloses fluid that was originally amniotic fluid. Along the edges of the neural folds lies the neural crest; from its cells ganglia will arise that lie next to the spinal cord, and its cells spread out over the body. To the left and right of the neural tube, paired blocks of mesoderm develop, called somites (soma = body; Fig. 24). They appear from the head end backwards, about 42 to 44 pairs in total, and the vertebrae, the muscles of the trunk and part of the skin of the back will develop from them. Because they form at a regular pace, the number of visible somites is used in embryology to describe the age of an embryo in this period.

The allantois

In the third week the yolk sac grows out, like a finger, into the connective stalk (Fig. 21 C2). This small organ is called the allantois (allantos = sausage). It is involved in the formation of the first blood vessels, and later its base takes part in the development of the bladder. In humans it remains small. In the embryos of birds and reptiles the allantois becomes a large sac that stores waste and, together with the chorion, serves for breathing through the eggshell.

The blood and its circulation

In the second week the embryonic disc is so small that nutrients and oxygen from the mother can reach the cells by diffusion through the fluid between the cells. Waste products are removed in the same way. The growth of the embryonic disc in the third week means that this is no longer sufficient, and a new transport system is needed. It is created from the middle of the third week on.

The first blood vessels and the first blood develop in the mesoderm on the outside of the yolk sac, in the connective stalk and around the allantois. Clusters of cells form there, called blood islands (Fig. 25). Cavities arise in the blood islands. The islands grow, connect with each other, and their cavities merge, so that capillaries are formed. These vessels become longer and find their way to the embryonic disc. Some cells in their wall become blood cells that float in the fluid inside the vessels.

Blood is first formed outside the embryonic disc. In the embryo itself, blood is not formed until about the fifth week, when the liver takes over this task.

The heart: flow and stagnation

The blood flows in fine vessels in the embryo on the ventral side towards the head, where its flow meets firmer tissue. It must turn around to flow back on the dorsal side of the embryonic disc to the connective stalk and the trophoblast. At this turning point the stream is dammed up and briefly stops. In this place, at the head end of the neural plate and in front of the tissues that will become the head, the heart is created (Fig. 26). It starts to beat around day 21 or 22.

In the phenomenological reading of these events, the embryo first shows a flow of liquid, and because the stream meets firmer tissue and is dammed up, the beating heart comes into being. Similar rhythms appear whenever a stream meets resistance: waves breaking on the coast, water flowing around stones in a brook, the rhythmic patterns of flow-forms. Seen in this way, the heart arises from flow and stagnation and is not the cause of the flow; it is the one place where the flowing blood comes to a standstill for a moment.

Mainstream embryology describes the same stage in terms of tissue: the heart develops from a horseshoe-shaped field of mesoderm at the head end, the cardiogenic area, whose paired tubes fuse into a single heart tube. Both descriptions agree on the place and the timing. They differ in what they take as the starting point: the phenomenological view begins with the movement of the fluid, the textbook account with the tissue that forms the wall.

The circulatory system is the first organ system in operation.

The chorion and the trophoblast

The rampant growth of the trophoblast slows down towards the end of the third week. Cords of cytotrophoblast grow through the syncytiotrophoblast and form an outer shell that binds the chorion to the uterine tissue. Mesoderm grows into the villi (= tufts), and blood vessels develop in them that connect via the connective stalk with the embryo. From the end of this week the exchange of substances with the blood of the mother takes place in these villi.

Characteristics

The focus of development in the third week is on the embryonic disc. There is still much growth in the periphery, but it is no longer a proliferating growth, and the periphery becomes relatively calm. If the growth movement of the second week continued, the human being would, in the words of this approach, stay in the periphery, in the enclosing organs, and would not develop a body.

From the periphery (yolk sac, connective stalk) the blood circulation arises and finds its centre in the heart. The heart arises in the stream of blood, and its pulsation arises from the alternation of standstill and flow.

At the same time the embryonic disc gains thickness through the mesoderm cells that come from the primitive streak and the primitive knot. This gives the disc content. Head and tail end, left and right are created by the notochord, and later reinforced by the neural tube. The embryonic disc has become a spatial, three-dimensional structure.

The third week is a decisive point in human development. Without the formation of circulation and heart the embryo cannot grow beyond the size that diffusion allows. What follows, the enclosing of the embryo by the amnion and the forming of the body, is described on the page about the fourth week.