section through the axis of the ovum). A, B, C three stages in the
formation of the blastula; D, E curving of the blastula; F complete
gastrula. h segmentation-cavity. g primitive gut-cavity.))

The gastrulation of the amphioxus is especially interesting because
this lowest and oldest of all the vertebrates is of the highest
significance in connection with the evolution of the vertebrate stem,
and therefore with that of man (compare Chapters 2.16 and 2.17). Just
as the comparative anatomist traces the most elaborate features in the
structures of the various classes of vertebrates to divergent
development from this simple primitive vertebrate, so comparative
embryology traces the various secondary forms of vertebrate
gastrulation to the simple, primary formation of the germinal layers
in the amphioxus. Although this formation, as distinguished from the
cenogenetic modifications of the vertebrate, may on the whole be
regarded as palingenetic, it is nevertheless different in some
features from the quite primitive gastrulation such as we have, for
instance, in the Monoxenia (Figure 1.29) and the Sagitta. Hatschek
rightly observes that the segmentation of the ovum in the amphioxus is
not strictly equal, but almost equal, and approaches the unequal. The
difference in size between the two groups of cells continues to be
very noticeable in the further course of the segmentation; the smaller
animal cells of the upper hemisphere divide more quickly than the
larger vegetal cells of the lower (Figure 1.38 A, B). Hence the
blastoderm, which forms the single-layer wall of the globular blastula
at the end of the cleavage-process, does not consist of homogeneous
cells of equal size, as in the Sagitta and the Monoxenia; the cells of
the upper half of the blastoderm (the mother-cells of the ectoderm)
are more numerous and smaller, and the cells of the lower half (the
mother-cells of the entoderm) less numerous and larger. Moreover, the