Fig.149. Longitudinal section of a human embryo of the fourth week, one-fifth of an inch long. Fig. 149—Longitudinal section of a human embryo of the fourth week, one-fifth of an inch long, magnified. (From _Kollmann._)
Fig.150. Transverse section of a human embryo of fourteen days. Fig. 151. Transverse section of a shark-embryo (or young selachius). Fig. 150—Transverse section of a human embryo of fourteen days. _mr_ medullary tube, _ch_ chorda. _vu_ umbilical vein, _mt_ myotome, _mp_ middle plate, _ug_ prorenal duct, _lh_ body-cavity, _e_ ectoderm, _bh_ ventral skin, _hf_ skin-fibre layer, _df_ gut-fibre layer. (From _Kollmann._) Fig. 151—Transverse section of a shark-embryo (or young selachius). _mr_ medullary tube, _ch_ chorda, _a_ aorta, _d_ gut, _vp_ principal (or subintestinal) vein, _mt_ myotome, _mm_ muscular mass of the provertebra, _mp_ middle plate, _ug_ prorenal duct, _lh_ body-cavity, _e_ ectoderm of the rudimentary extremities, _mz_ mesenchymic cells, _z_ point where the myotome and nephrotome separate. (From _H. E. Ziegler._)
Fig.152. Transverse section of a duck-embryo with twenty-four primitive segments. Fig. 152—Transverse section of a duck-embryo with twenty-four primitive segments. (From _Balfour._) From a dorsal lateral joint of the medullary tube (_spc_) the spinal ganglia (_spg_) grow out between it and the horn-plate. _ch_ chorda, _ao_ double aorta, _hy_ gut-gland layer, _sp_ gut-fibre layer, with blood-vessels in section, _ms_ muscle plate, in the dorsal wall of the myocœl (episomite). Below the cardinal vein (_cav_) is the prorenal duct (_wd_) and a segmental prorenal canal (_st_). The skin-fibre layer of the body-wall (_so_) is continued in the amniotic fold (_am_). Between the four secondary germinal layers and the structures formed from them there is formed embryonic connective matter with stellate cells and vascular structures (Hertwig’s “mesenchym”).
Chapter XIV. THE ARTICULATION OF THE BODY[26]
[26] The term articulation is used in this chapter to denote both “segmentation” and “articulation” in the ordinary sense.—Translator.
The vertebrate stem, to which our race belongs as one of the latest and most advanced outcomes of the natural development of life, is rightly placed at the head of the animal kingdom. This privilege must be accorded to it, not only because man does in point of fact soar far above all other animals, and has been lifted to the position of “lord of creation”; but also because the vertebrate organism far surpasses all the other animal-stems in size, in complexity of structure, and in the advanced character of its functions. From the point of view of both anatomy and physiology, the vertebrate stem outstrips all the other, or invertebrate, animals.