In the narrow middle part the first three primitive segments may be seen.)

Frontal (or horizontal-longitudinal) section of a triton-embryo with three pairs of primitive segments.

The body is a series of metamera, as several of the primitive segments are developed.

Transverse section of a duck-embryo with twenty-four primitive segments.

In the middle of the sole-shaped embryonic shield the first primitive segments immediately begin to make their appearance.

The number of primitive segments increases, and generally amounts to fourteen, some forty-eight to fifty hours after impregnation.

Hence we shall give the name of "somites" or primitive segments to these so-called "primitive vertebrae."

These pairs of "cubes" of the mesoderm are the first traces of the primitive segments or somites, the so-called "protovertebrae."

In Figure 1.158 only the outlines of the eight primitive segments are indicated, in Figure 1.159 their cavities and muscular walls.

Figure 1.79 at the commencement of coelom formation (still without segments), Figure 1.80 at the stage with four primitive segments.

The foremost of these primitive segments (us1) is the first and oldest; in Figures 1.124 and 1.157 there are already five formed.