Plant and animal: two ways of embryonic development

On this page the embryonic development of a flowering plant and of a simple animal, a sea urchin, are described and compared from a Goethean point of view, also known as phenomenology. The comparison brings out the fundamental difference between the shape and essence of a plant and of an animal, and it forms the background for the human development described on the following pages of this embryology series.

The development of the embryo of the plant

When plants flower, pollen is carried from the stamens to the stigma of the pistil by wind, insects or other animals. The pollen grain makes a tube through the style of the pistil to the egg cell in the ovary. Fertilisation takes place and then follows the first division (see Fig. 1, stage 2), by which a small apical cell (A) and a large basal cell (B) are formed. The apical cell divides into four cells and forms a small ball. The basal cell divides as well and forms a short row of cells beneath it (stage 3). The apical clump of cells grows and forms a spherical ball. The lower part with the basal cell stops growing and dividing quite soon (stages 4 and 5). This part is called the suspensor (C). Stage 5 is called the globular stage of the embryo.

The apical tissue then grows sideways (stage 6), and the cotyledons (D) are formed from it. At the same time the tissue between the cotyledons and the suspensor differentiates into the growing point of the root (the apical root meristem, E), the growing point of the shoot (the apical shoot meristem, F) and the connecting vascular tissue (G), stages 7 and 8. The cotyledons grow and fold out. The seed, for example a peanut, is formed. In plants with two cotyledons the embryo passes through shapes that botanists call the heart stage and the torpedo stage, after the outline the embryo has at those moments.

The seed matures and goes into rest. It starts to grow again only when it comes into the ground and the conditions are favourable for germination. One might speak of a "double fertilisation" in an imaginative sense: first the pollen fertilises the egg, then the seed has to fall or be sown into the earth. The seed is received by the earth. (Botany uses the term double fertilisation for something else as well: in flowering plants a second sperm nucleus fuses with other cells in the ovule and forms the endosperm, the nutritive tissue of the seed.)

The following processes are visible

• right from the first division the embryo is growing;
• immediately there is cell differentiation, the apical and basal cells are different;
• the tissue is solid.

The development of the embryo of a simple animal (e.g. a sea urchin)

At fertilisation an egg cell and a sperm cell fuse and create a zygote (= fertilised egg). A little later the first division takes place: two equally big cells are created, each half the size of the zygote. The divisions follow one another in quick succession. The embryo is and stays spherical and does not grow; the cells simply become smaller with every division. The stage of 16 to 64 cells is called a morula (= mulberry) (see Fig. 2, the fifth stage, side view).

Then the cells arrange themselves at the periphery and a cavity forms in the middle. This cavity is filled with fluid. The embryo is now called a blastula (Fig. 2, the sixth stage, a cross section). In sea urchins the blastula grows cilia, starts to rotate within its envelope and then hatches and swims freely.

The cell divisions in the wall of the blastula continue, and then a part of the wall bends inwards to make a tube to the inside. This indentation looks as if a finger were pushed into a soft ball. It begins at the spot called the blastopore (= opening of the vesicle) (Fig. 2, stages 7 and 8). The embryo is now called a gastrula (gaster = stomach). The indentation continues until it reaches the opposite wall. There the tissue breaks open. The blastopore becomes the anus and the new breakthrough forms the mouth. Between these two a tube is formed that will become the digestive tract. Between the digestive tract and the outer wall or skin a body cavity is formed, in which clumps of cells, the mesoderm, arise from which the organs will develop. The young sea urchin then becomes a larva, the pluteus, which floats in the plankton for weeks before it changes into the familiar spiny adult.

Three types of tissue are formed

• an outer membrane or outer skin, the ectoderm (ecto = outside and derma = skin, blue);
• an inner membrane or inner skin, the entoderm (also called endoderm; ento = inside, green);
• and between them a cavity in which there is in-between tissue, the mesoderm (mesos = middle, red).

These three tissues are called the three germ layers. Later on all tissues and organs grow out of them.

Two cavities come into being

• the digestive tract: an external cavity, a piece of the outside world taken into the body, which stays open to the world outside through the mouth and the anus;
• the body cavity: the cavity between the skin and the digestive tract. This cavity has no connection to the world outside. In it all organs and the centre of the body develop.

The following processes are visible

• in the beginning there is no growth;
• in the beginning there is no cell differentiation;
• two different cavities are formed.

Differences between plants and animals

The embryonic development of plants and animals is different. Plants show immediate growth, animals do not at first. The cells of plants differ immediately, whereas this is not the case with animals. Plants are solid and animals have two cavities. Animals bend inwards and make a body cavity; plants do not form an inner space.

This can also be seen later on. Plants grow at the ends of their twigs, shoots and roots, directed to the surroundings. The growing points are always placed at the ends of twigs, stems and roots, in the periphery. Processes take place on the outside. Growth rate and size are largely determined by the environment. A plant on poor sandy soil grows less well than a plant on nutrient-rich clay. Plants live in their environment, spread out into it.

Animals grow to a certain, somewhat variable size and then stop growing. They are not influenced by the environment as much as plants. Plants are fixed in the earth, animals can move. Plants have neither an inner cavity nor a centre; animals do, in the heart. Animals have developed an interior space in which processes take place, and they have an inner life. See Fig. 3.

In anthroposophy the inner life of animals is linked to the inner space of the body cavity. In that view the cavity is the basis for the astral body, the non-physical body in which the soul capacities of observing and thinking, feeling, and willing and doing take place. Plants, in this view, have neither a soul nor a body cavity, while plants and animals both have an ether or life body (see the four ethers). Plants can grow freely, animals cannot, because they are oriented towards their centre, as the gastrula already shows. Table 1 summarises the differences.

Open and closed forms

The two ways of development lead to two basic forms. A plant is an open form. It never reaches a final shape but keeps adding leaves, branches and roots for as long as it lives, and a cutting of a willow can grow into a complete new tree. An animal is a closed form. Its body plan is laid down early, its organs lie inside a skin that separates it from the world, and a severed limb of a mammal does not grow into a new animal.

The sea urchin shows the first step of that closing: a hollow ball that turns part of its surface inwards. What was outside becomes inside, and the animal now has a world of its own within its skin. Mouth and anus remain the gates through which that inner world is connected with the outer one.

The same gesture in the human embryo

The human embryo follows the animal way, with its own variations. In the first week the fertilised egg divides without growing and without much differentiation, just as in the sea urchin. Only when the embryo has settled in the wall of the uterus does real growth begin. Then cavities are formed: the amniotic cavity and the yolk sac in the second week, the body cavity and the gut tube in the third and fourth weeks. The contrast between a plant, which grows outward into its surroundings, and an animal, which forms an inner space around a centre, is therefore a useful key for reading the pages that follow, from the first week to the fourth week, when the heart begins to beat.