In order to understand properly the real nature and origin of articulation in the human body and that of the higher vertebrates, it is necessary to compare it with that of the lower vertebrates, and bear in mind always the genetic connection of all the members of the stem. In this the simple development of the invaluable amphioxus once more furnishes the key to the complex and cenogenetically modified embryonic processes of the craniota. The articulation of the amphioxus begins at an early stage—earlier than in the craniotes. The two cœlom-pouches have hardly grown out of the primitive gut (Fig. 156 _c_) when the blind fore part of it (farthest away from the primitive mouth, _u_) begins to separate by a transverse fold (_s_): this is the first primitive segment. Immediately afterwards the hind part of the cœlom-pouches begins to divide into a series of pieces by new transverse folds (Fig. 157). The foremost of these primitive segments (_us_1) is the first and oldest; in Figs. 124 and 157 there are already five formed. They separate so rapidly, one behind the other, that eight pairs are formed within twenty-four hours of the beginning of development, and seventeen pairs twenty-four hours later. The number increases as the embryo grows and extends backwards, and new cells are formed constantly (at the primitive mouth) from the two primitive mesodermic cells (Figs. 159–160). Figs. 161 and 162. Transverse section of shark-embryos (through the region of the kidneys). Figs. 161 and 162—Transverse section of shark-embryos (through the region of the kidneys). (From _Wijhe_ and _Hertwig._) In Fig. 162 the dorsal segment-cavities (_h_) are already separated from the body-cavity (_lh_), but they are connected a little earlier (Fig. 161), _nr_ neural tube, _ch_ chorda, _sch_ subchordal string, _ao_ aorta, _sk_ skeletal-plate, _mp_ muscle-plate, _cp_ cutis-plate, _ w_ connection of latter (growth-zone), _vn_ primitive kidneys, _ug_ prorenal duct, _uk_ prorenal canals, _ us_ point where they are cut off, _tr_ prorenal funnel, _ mk_ middle germ-layer (_mk_1 parietal, _ mk_2 visceral), _ik_ inner germ-layer (gut-gland layer).