Almost simultaneously with the formation of the mouth the first gill-cleft breaks through in the fore section of the Amphioxus-embryo (generally forty hours after the commencement of development). It now begins to nourish itself independently, as the food material stored up in the ovum is completely used up. The further development of the free larvæ takes place very slowly, and extends over several months. The body becomes much longer, and is compressed at the sides, the head-end being broadened in a sort of triangle. Two rudimentary sense-organs are developed in it. Inside we find the first blood-vessels, an upper or dorsal vessel, corresponding to the aorta, between the gut and the dorsal cord, and a lower or ventral vessel, corresponding to the subintestinal vein, at the lower border of the gut. Now, the gills or respiratory organs also are formed at the fore-end of the alimentary canal. The whole of the anterior or respiratory section of the gut is converted into a gill-crate, which is pierced trellis-wise by numbers of branchial-holes, as in the ascidia. This is done by the foremost part of the gut-wall joining star-wise with the outer skin, and the formation of clefts at the point of connection, piercing the wall and leading into the gut from without. At first there are very few of these branchial clefts; but there are soon a number of them—first in one, then in two, rows. The foremost gill-cleft is the oldest. In the end we have a sort of lattice work of fine gill-clefts, supported on a number of stiff branchial rods; these are connected in pairs by transverse rods. Figs. 222-224. Transverse sections of young Amphioxus-larvae. Figs. 222–224—Transverse sections of young Amphioxus-larvæ (diagrammatic, from _ Ralph._) (Cf. also Fig. 216.) In Fig. 222 there is free communication from without with the gut-cavity (_D_) through the gill-clefts (_K_). In Fig. 223 the lateral folds of the body-wall, or the gill-covers, which grow downwards, are formed. In Fig. 224 these lateral folds have united underneath and joined their edges in the middle line of the ventral side (_R_ seam). The respiratory water now passes from the gut-cavity (_D_) into the mantle-cavity (_A_). The letters have the same meaning throughout: _N_ medullary tube, _Ch_ chorda, _ M_ lateral muscles, _Lh_ body-cavity, _G_ part of the body-cavity in which the sexual organs are subsequently formed. _ D_ gut-cavity, clothed with the gut-gland layer (_a_). A mantle-cavity, _K_ gill-clefts, _b_=_E_ epidermis, _E_1 the same as visceral epithelium of the mantle-cavity, _E_2 as parietal epithelium of the mantle-cavity.