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.

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,