archigastrula

There is to-day only the lowest of the vertebrates, the amphioxus, that exhibits the original form of those processes, or the palingenetic gastrulation which we have considered in the preceding chapter, and which culminates in the formation of the archigastrula (Figure 1.38).

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

The chief of these stages are the spherical blastula (in its simplest primary form), and the succeeding archigastrula, the pure, original form of the gastrula which the

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

In consequence of these considerable variations arising in the course of the gastrulation, the primitive uni-axial form of the archigastrula in the amphioxus has already become tri-axial, and thus the two-sidedness, or bilateral symmetry, of the vertebrate body has already been determined.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

We give the name of the "bell-gastrula," or archigastrula, to the gastrula that succeeds it.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the Monoxenia which I described (Chapter 1.8, Figure 1.29).

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

Apart from this bilateral structure, the gastrula of the amphioxus resembles the typical archigastrula of the lower animals (Figures 1.30 to 1.36) in developing the two primary germinal layers from a single layer of cells.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

The primitive mouth, which at first, in the typical archigastrula, lay at the vegetal pole of the main axis, is forced away to the dorsal side; and whereas its two lips lay at first in a plane at right angles to the chief axis, they are now so far thrust aside that their plane cuts the axis at a sharp angle.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

The pure type of the original gastrula (or archigastrula, Figure 1.29 I) is seen in the

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

The pure archigastrula of the latter forms is uni-axial, and it is round in its whole length in transverse section.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

As in all the other animals that form an archigastrula, the whole body is nothing but a simple gastric sac or stomach; its internal cavity is the primitive gut (progaster or archenteron, Figure 1.38 g, 1.35 d), and its aperture the primitive mouth (prostoma or blastoporus, o).

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876