Fig. 46—Embryonic vesicle of triton (blastula), outer view, with the transverse fold of the primitive mouth (u). (From Hertwig.)
This reduction becomes easier if, after considering the gastrulation of the tailless amphibia (frogs and toads), we glance for a moment at that of the tailed amphibia, the salamanders. In some of the latter, that have only recently been carefully studied, and that are phylogenetically older, the process is much simpler and clearer than is the case with the former and longer known. Our common water-salamander (Triton taeniatus) is a particularly good subject for observation. Its nutritive yelk is much smaller and its formative yelk less obscured with black pigment-cells than in the case of the frog; and its gastrulation has better retained the original palingenetic character. It was first described by Scott and Osborn (1879), and Oscar Hertwig especially made a careful study of it (1881), and rightly pointed out its great importance in helping us to understand the vertebrate development. Its globular blastula (Fig. 45) consists of loosely-aggregated, yelk-filled entodermic cells or yelk-cells (dz) in the lower vegetal half; the upper, animal half encloses the hemispherical segmentation-cavity (fh), the curved roof of which is formed of two or three strata of small ectodermic cells. At the point where the latter pass into the former (at the equator of the globular vesicle) we have the border zone (rz). The folding which leads to the formation of the gastrula takes place at a spot in this border zone, the primitive mouth (Fig. 46 u).
Fig. 47—Sagittal section of a hooded-embryo (depula) of triton (blastula at the commencement of gastrulation). ak outer germinal layer, ik inner germinal layer, fh segmentation-cavity, ud primitive gut, u primitive mouth, dl and vl dorsal and ventral lips of the mouth, dz yelk-cells. (From Hertwig.)
Unequal segmentation takes place in some of the cyclostoma and in the oldest fishes in just the same way as in most of the amphibia. Among the cyclostoma (“round-mouthed”) the familiar lampreys are particularly interesting. In respect of organisation and development they are half-way between the acrania (lancelet) and the lowest real fishes (Selachii); hence I divided the group of the cyclostoma in 1886 from the real fishes with which they were formerly associated, and formed of them a special class of vertebrates. The ovum-segmentation in our common river-lamprey (Petromyzon fluviatilis) was described by Max Schultze in 1856, and afterwards by Scott (1882) and Goette (1890).
Unequal total segmentation follows the same lines in the oldest fishes, the selachii and ganoids, which are directly descended from the cyclostoma. The primitive fishes (Selachii), which we must regard as the ancestral group of the true fishes, were generally considered, until a short time ago, to be discoblastic. It was not until the beginning of the twentieth century that Bashford Dean made the important discovery in Japan that one of the oldest living fishes of the shark type (Cestracion japonicus) has the same total unequal segmentation as the amphiblastic plated fishes (ganoides).[[22]] This is particularly interesting in connection with our subject, because the few remaining survivors of this division, which was so numerous in paleozoic times, exhibit three different types of gastrulation.
[22] Bashford Dean, Holoblastic Cleavage in the Egg of a Shark, Cestracion japonicus Macleay. Annotationes zoologicae japonenses, vol. iv, Tokio, 1901.
