At an early stage an opaque spot is seen in the middle of the clear germinative area, and this also passes from a circular to an oval shape. At first this shield-shaped marking is very delicate and barely perceptible; but it soon becomes clearer, and now stands out as an oval shield, surrounded by two rings or areas (Fig. 117). The inner and brighter ring is the remainder of the pellucid area, and the dark outer ring the remainder of the opaque area; the opaque shield-like spot itself is the first rudiment of the dorsal part of the embryo. We give it briefly the name of embryonic shield or dorsal shield. In most works this embryonic shield is described as “the first rudiment or trace of the embryo,” or “primitive embryo.” But this is wrong, though it rests on the authority of Baer and Bischoff.
Fig. 119—Median longitudinal section of the gastrula of four vertebrates. (From Rabl.) A discogastrula of a shark (Pristiurus). B amphigastrula of a sturgeon (Accipenser). C amphigastrula of an amphibium (Triton). D epigastrula of an amniote (diagram). a ventral, b dorsal lip of the primitive mouth.
As a matter of fact, we already have the embryo in the stem-cell, the gastrula, and all the subsequent stages. The embryonic shield is simply the first rudiment of the dorsal part, which is the earliest to develop. As the older names of “embryonic rudiment” and “germinative area” are used in many different senses—and this has led to a fatal confusion in embryonic literature—we must explain very clearly the real significance of these important embryonic parts of the amniote. It will be useful to do so in a series of formal principles:—
1. The so-called ”first trace of the embryo” in the amniotes, or the embryonic shield, in the centre of the pellucid area, consists merely of an early differentiation and formation of the middle dorsal parts.
2. Hence the best name for it is ”the dorsal shield,” as I proposed long ago.
3. The germinative area, in which the first embryonic blood-vessels appear at an early stage, is not opposed as an external area to the ”embryo proper,” but is a part of it.
4. In the same way, the yelk-sac or the umbilical vesicle is not a foreign external appendage of the embryo, but an outlying part of its primitive gut.
5. The dorsal shield gradually separates from the germinative area and the yelk-sac, its edges growing downwards and folding together to form ventral plates.