The gill-clefts, which originally in the older acrania pierced the wall of the fore-gut, and the gill-arches that separated them, were presumably also segmental, and distributed among the various metamera of the chain, like the gonads in the after-gut and the nephridia. In the amphioxus, too, they are still segmentally formed. Probably there was a division of labour of the hyposomites in the older (and long extinct) acrania, in such wise that those of the fore-gut took over the function of breathing and those of the after-gut that of reproduction. The former developed into gill-pouches, the latter into sex-pouches. There may have been primitive kidneys in both. Though the gills have lost their function in the higher animals, certain parts of them have been generally maintained in the embryo by a tenacious heredity. At a very early stage we notice in the embryo of man and the other amniotes, at each side of the head, the remarkable and important structures which we call the gill-arches and gill-clefts (Figs. 167–170 _f_). They belong to the characteristic and inalienable organs of the amniote-embryo, and are found always in the same spot and with the same arrangement and structure. There are formed to the right and left in the lateral wall of the fore-gut cavity, in its foremost part, first a pair and then several pairs of sac-shaped inlets, that pierce the whole thickness of the lateral wall of the head. They are thus converted into clefts, through which one can penetrate freely from without into the gullet. The wall thickens between these branchial folds, and changes into an arch-like or sickle-shaped piece—the gill, or gullet-arch. In this the muscles and skeletal parts of the branchial gut separate; a blood-vessel arch rises afterwards on their inner side (Fig. 98 _ka_). The number of the branchial arches and the clefts that alternate with them is four or five on each side in the higher vertebrates (Fig. 170 _d, f, f′, f″_). In some of the fishes (selachii) and in the cyclostoma we find six or seven of them permanently.
Fig.173. Transverse section of the pelvic region and hind legs of a chick-embryo of the fourth day. Fig. 173—Transverse section of the pelvic region and hind legs of a chick-embryo of the fourth day, magnified. _h_ horn-plate, _w_ medullary tube, _n_ canal of the tube, _u_ primitive kidneys, _x_ chorda, _e_ hind legs, _b_ allantoic canal in the ventral wall, _t_ aorta, _ v_ cardinal veins, _a_ gut, _d_ gut-gland layer, _ f_ gut-fibre layer, _g_ embryonic epithelium, _r_ dorsal muscles, _c_ body-cavity or cœloma. (From _ Waldeyer._)