(FIGURES 2.222 TO 2.224. Transverse sections of young Amphioxus-larvae (diagrammatic, from Ralph.) (Cf. also Figure 2.216.) In Figure 2.222 there is free communication from without with the gut-cavity (D) through the gill-clefts (K). In Figure 2.223 the lateral folds of the body-wall, or the gill-covers, which grow downwards, are formed. In Figure 2.224 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, E1 the same as visceral epithelium of the mantle-cavity, E2 as parietal epithelium of the mantle-cavity.) At an early stage of embryonic development the structure of the Amphioxus-larva is substantially the same as the ideal picture we have previously formed of the "Primitive Vertebrate" (Figures 1.98 to 1.102). But the body afterwards undergoes various modifications, especially in the fore-part. These modifications do not concern us, as they depend on special adaptations, and do not affect the hereditary vertebrate type. When the free-swimming Amphioxus-larva is three months old, it abandons its pelagic habits and changes into the young animal that lives in the sand. In spite of its smallness (one-eighth of an inch), it has substantially the same structure as the adult. As regards the remaining organs of the Amphioxus, we need only mention that the gonads or sexual glands are developed very late, immediately out of the inner cell-layer of the body-cavity. Although we can find afterwards no continuation of the body-cavity (Figure 2.216 U) in the lateral walls of the mantle-cavity, in the gill-covers or mantle-folds (Figure 2.224 U), there is one present in the beginning (Figure 2.224 Lh). The sexual cells are formed below, at the bottom of this continuation (Figure 2.224 S). For the rest, the subsequent development into the adult Amphioxus of the larva we have followed is so simple that we need not go further into it here. We may now turn to the embryology of the Ascidia, an animal that seems to stand so much lower and to be so much more simply organised, remaining for the greater part of its life attached to the bottom of the sea like a shapeless lump. It was a fortunate accident that Kowalevsky first examined just those larger specimens of the Ascidiae that show most clearly the relationship of the vertebrates to the invertebrates, and the larvae of which behave exactly like those of the Amphioxus in the first stages of development. This resemblance is so close in the main features that we have only to repeat what we have already said of the ontogenesis of the Amphioxus.