of growth of the epiblast,
as shown by the unbarbed arrow, over the yolk. This stage is called
the gastrula stage; ar. is the cavity of the gastrula, the archenteron;
b.p. is its opening or blastopore. Such a gastrula, formed mainly by
overgrowth of the epiblast, is called an epibolic gastrula, as
distinguished from the invaginate gastrula of amphioxus. The
difference is evidently entirely due to the presence of yolk, and the
consequent modification of invagination in the former case.

Section 9. Comparing the two gastrulas, it is not difficult to see that if
we imagine the ventral wall of the archenteron of amphioxus to have
its cells enormously enlarged through the mixing of yolk with their
protoplasm, we should have a gastrula essentially like that of the frog.

Section 10. Figure 6 shows a slightly later ovum than Figure 5, seen
from the dorsal side. b.p. is the blastopore. In front of that appears a
groove, the neural groove, bordered on either side by a ridge, the
neural fold (n.f.). This is seen in section in Figure 7; s.c. is the neural
groove; n.f., as before, the neural fold. The neural folds ultimately bend
over and meet above, so that s.c. becomes a canal, and is finally
separated from the epiblast to form the spinal cord. Below the neural
groove a thickening of the dorsal wall of the archenteron appears, and
is pinched off to form a longitudinal rod, the precursor of the vertebral
column, the notochord, shown in Figure 7 (n.c.), as imperfectly
pinched off.

Section 11. Simultaneously, on either side of the notochord appear a
series of solid masses of cells, derived mainly by cell division from the
cells of the wall of the archenteron, and filling up and obliterating the
segmentation cavity. These masses increase in number by the
addition of fresh ones behind, during development, and are visible in
the dorsal view as brick-like masses, the mesoblastic somites or
proto-vertebrae (Figure 6, i., ii., iii.). In Figure 7, these masses are
indicated by dotting. In such a primitive type as amphioxus these
mesoblastic -somites- [masses] contain a cavity, destined to be the
future body cavity, from the first. In the frog, the cavity is not at first
apparent; the mesoblast at first seems quite solid, but subsequently
what is called the splitting of the mesoblast occurs, and the body
cavity (b.c. in Figure 7) appears. The outer mesoblast, lying
immediately under the epiblast, constitutes the substance of the