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” (Figs. 98–102). 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 (Fig. 216 _U_) in the lateral walls of the mantle-cavity, in the gill-covers or mantle-folds (Fig. 224 _U_), there is one present in the beginning (Fig. 224 _Lh_). The sexual cells are formed below, at the bottom of this continuation (Fig. 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 Ascidiæ that show most clearly the relationship of the vertebrates to the invertebrates, and the larvæ 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. The ovum of the larger Ascidia (_Phallusia, Cynthia,_ etc.) is a simple round cell of 1/250 to 1/125 of an inch in diameter. In the thick fine-grained yelk we find a clear round germinal vesicle of about 1/750 of an inch in diameter, and this encloses a small embryonic spot or nucleolus. Inside the membrane that surrounds the ovum, the stem-cell of the Ascidia, after fecundation, passes through just the same metamorphoses as the stem-cell of the Amphioxus. It undergoes total segmentation; it divides into two, four, eight, sixteen, thirty-two cells, and so on. By continued total cleavage the morula, or mulberry-shaped cluster of cells, is formed. Fluid gathers inside it, and thus we get once more a globular vesicle (the blastula); the wall of this is a single stratum of cells, the blastoderm. A real gastrula (a simple bell-gastrula) is formed from the blastula by invagination, in the same way as in the amphioxus.