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. 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).