Embryology: Images of Man – an introduction

Embryology describes how a human being develops from a single fertilised cell into a body with organs, limbs and a face. The pages in this series follow that development week by week. They begin with the principles of embryonic development in the plant and in a simple animal, because the contrast between the two sheds light on what happens later in the human embryo, and they are written from the point of view of phenomenology.

Sources and background

The basis of the series is formed by a number of books by the anatomist Blechschmidt and by Wilmar, and by the work of the Dutch embryologist Jaap van der Wal. Blechschmidt described the embryo as a whole that shapes itself through growth movements, and that picture of development as movement runs through all the pages. Unless otherwise specified the drawings are based on Moore's "Before We Are Born", a standard textbook of human embryology.

Why embryology?

One may wonder why a subject such as embryology is worth studying for someone who is not going to work in medicine. Primarily it is about the development of the embryo itself. From that development it becomes clear that a human being is a human being from conception onwards, only in a different form than the one met after birth. Seeing this may lead to respect for unborn life, which is a second reason to study it.

A third reason is that many processes occur simultaneously. While the germ layers form, the membranes around the embryo grow, the blood vessels appear and the body folds. Holding all of these in consciousness at the same time is an exercise in itself. In addition, the development is a three-dimensional process that has to be visualised from two-dimensional drawings, which is an appeal to the imagination. This makes embryology a difficult subject, and precisely for that reason a stimulating one for the development of thought.

A way of looking

The embryonic development of the human being is described here and represented in a large number of drawings, so that an image of the development can arise. The starting point for the description is the anthroposophical view of the human being, which sheds a particular light on embryology; where the pages speak from that view, they say so. The anatomical facts themselves are those of ordinary textbooks.

The development is not described in an anatomical or analytical way. Instead the aim is to find the inner movement, the coherence and the larger context by observing the changing forms and processes. This is more about understanding the language of the forms of living organisms than about a causal explanation of these forms. It aims at an awareness of what Goethe called the "transcendental" quality of life, and of the human being. The same approach is used elsewhere on this site for animals; the introduction to the phenomenological approach explains it in more general terms.

An analytical description asks which gene or which signal causes a structure. A phenomenological description asks what the structure does in the whole, in which direction it moves, and how it relates to what came before and what follows. Both are legitimate questions. The second kind needs the first as its material: without accurate observation there is nothing to read.

Who the pages are written for

The pages have been written for the interested layman. On several occasions matters have been simplified. Anyone who wants to go further will find manuals and an informative website (www.embryology.ch) with drawings and photographs. For more background the website of the embryologist Jaap van der Wal (www.embryo.nl) may be consulted, as well as the publications listed among the sources.

Colours in the drawings

In the drawings, where appropriate, four colours are used: blue for the ectoderm, red for the mesoderm, green for the entoderm and yellow for the allantois. The ectoderm is the outer germ layer; the nervous system, the sense organs and the outer skin develop from it. The mesoderm is the middle layer, from which muscles, bones, connective tissue, the blood and the heart arise. The entoderm is the inner layer that lines the digestive tract and forms glands such as the liver and the pancreas. The allantois is a small outpocketing of the yolk sac whose vessels become part of the umbilical cord.

Keeping these colours in mind makes the drawings much easier to follow. When a structure suddenly turns red in a later drawing, it means that new mesoderm has appeared there.

The course of the series

The series follows the order of the development itself. First comes a comparison of plant and animal: the embryo of a flowering plant grows and differentiates from its first division onwards, whereas the embryo of a sea urchin first divides without growing and then folds inwards to make a digestive tract and a body cavity. That contrast between growing outwards and turning inwards returns in the human embryo.

Then the two cells at the beginning are described: the human egg cell and sperm, which are in almost every respect each other's opposite. The page on the conception describes how these two cells meet and fuse. After that come the weeks. In the first week the fertilised egg divides into ever smaller cells without growing, while it drifts through the oviduct; at the end of that week it hatches from its shell. In the second week it settles in the wall of the uterus and forms two cavities, the amniotic cavity and the yolk sac, with a flat disc of cells between them. In the third week the embryo gets direction: a head end and a tail end, a left and a right side, a front and a back appear, and the third germ layer is formed. In the fourth week the flat disc folds into a curved body with a head, a heart that has begun to beat and the first buds of the limbs.

A short timeline

For orientation, the main steps of the first month in textbook terms are these. Fertilisation takes place in the oviduct, usually within a day after ovulation. In the following days the zygote divides into two, four and eight cells and becomes a morula, a small ball of cells. Around the fifth day a fluid-filled cavity appears and the embryo is called a blastocyst, with an outer layer, the trophoblast, and an inner cell mass, the embryoblast. At the end of the first week the blastocyst leaves its protective shell, the zona pellucida, and begins to implant in the wall of the uterus.

During the second week the embryoblast forms a two-layered disc, and the amniotic cavity and the yolk sac appear on either side of it. The trophoblast grows into the uterine wall and forms the beginning of the placenta. In the third week gastrulation takes place: through the primitive streak cells migrate between the two layers and form the mesoderm, so that the disc becomes three-layered. The notochord and the neural plate appear. Around the end of the third week the first blood vessels and the tubular heart form, and early in the fourth week the heart begins to beat. In the fourth week the neural tube closes, the embryo folds into a cylindrical shape, and the arm buds and then the leg buds appear. By the end of the eighth week all the main organs have been laid down, and the embryonic period passes into the foetal period, in which the child mainly grows and matures.

Reading the drawings

Most of the drawings are cross-sections or longitudinal sections: the embryo is imagined as cut through, and one looks at the cut surface. A single section never shows the whole. It helps to picture how a series of such sections fits together, or to model the forms with clay or paper, so that the folding of flat layers into tubes and cavities becomes a movement that can be followed. Drawings of successive days are best read side by side. What looks like a new structure in one picture is often an old one that has grown, shifted or turned, and the question to ask each time is where it came from and where it is moving to.

The sizes matter as well. In the first two weeks the embryo is smaller than a grain of sand, and at the end of the fourth week it is still only a few millimetres long. Everything described in these pages happens at a scale that cannot be seen with the naked eye, and yet the basic plan of the body is laid down in that time.

The whole before the parts

A theme that runs through the whole series is that the whole comes before the parts. The embryo is not assembled from organs that are made separately and then joined; it is one living entity from the first cell onwards, which differentiates itself into regions, layers and organs. In that sense the development shows in its own way what the phenomenological approach looks for everywhere: the form as an expression of a whole. Read that way, each of the following pages describes one step in which that whole takes on a new shape.