stem-form of the vertebrates. We must assume as the common ancestral
group of both stems an extinct family of the extensive vermalia-stem,
the Prochordonia or Prochordata ("primitive chorda-animals").

In order to appreciate fully this remarkable fact, and especially to
secure the sound basis we seek for the genealogical tree of the
vertebrates, it is necessary to study thoroughly the embryology of
both these animals, and compare the individual development of the
Amphioxus step by step with that of the Ascidia. We begin with the
ontogeny of the Amphioxus.

From the concordant observations of Kowalevsky at Naples and Hatschek
at Messina, it follows, firstly, that the ovum-segmentation and
gastrulation of the Amphioxus are of the simplest character. They take
place in the same way as we find them in many of the lower animals of
different invertebrate stems, which we have already described as
original or primordial; the development of the Ascidia is of the same
type. Sexually mature specimens of the Amphioxus, which are found in
great quantities at Messina from April or May onwards, begin as a rule
to eject their sexual products in the evening; if you catch them about
the middle of a warm night and put them in a glass vessel with
seawater, they immediately eject through the mouth their accumulated
sexual products, in consequence of the disturbance. The males give out
masses of sperm, and the females discharge ova in such quantity that
many of them stick to the fibrils about their mouths. Both kinds of
cells pass first into the mantle-cavity after the opening of the
gonads, proceed through the gill-clefts into the branchial gut, and
are discharged from this through the mouth.

The ova are simply round cells. They are only 1/250 of an inch in