If we sum up the results of our anatomic study of the Amphioxus, and compare them with the familiar organisation of man, we shall find an immense distance between the two. As a fact, the highest summit of the vertebrate organisation which man represents is in every respect so far above the lowest stage, at which the lancelet remains, that one would at first scarcely believe it possible to class both animals in the same division of the animal kingdom. Nevertheless, this classification is indisputably just. Man is only a more advanced stage of the vertebral type that we find unmistakably in the Amphioxus in its characteristic features. We need only recall the picture of the ideal Primitive Vertebrate given in a former chapter, and compare it with the lower stages of human embryonic development, to convince ourselves of our close relationship to the lancelet. (Cf. Chapter XI) It is true that the Amphioxus is far below all other living vertebrates. It is true that it has no separate head, no developed brain or skull, the characteristic feature of the other vertebrates. It is (probably as a result of degeneration) without the auscultory organ and the centralised heart that all the others have; and it has no fully-formed kidneys. Every single organ in it is simpler and less advanced than in any of the others. Yet the characteristic connection and arrangement of all the organs is just the same as in the other vertebrates. All these, moreover, pass, during their embryonic development, through a stage in which their whole organisation is no higher than that of the Amphioxus, but is substantially identical with it. Fig.219. Transverse section of the head of the Amphioxus. Fig. 219—Transverse section of the head of the Amphioxus (at the limit of the first and second third of the body). (From _Boveri_) _a_ aorta (here double), _b_ atrium, _c_ chorda, _co_ umlaut cœloma (body-cavity), _e_ endostyl (hypobranchial groove), _g_ gonads (ovaries), _kb_ gill-arches, _kd_ branchial gut, _l_ liver-tube (on the right, one-sided), _m_ muscles, _n_ renal canals, _r_ spinal cord, _sn_ spinal nerves, _sp_ gill-clefts.