is true that the fully-developed Ascidia resembles the Amphioxus in
several important features of its internal structure, and especially
in the peculiar character of the gill-crate and gut. But in most other
features of organisation it is so far removed from it, and is so
unlike it in external appearance, that the really close relationship
of the two was not discovered until their embryology was studied. We
will now compare the embryonic development of the two animals, and
find to our great astonishment that the same embryonic form develops
from the ovum of the Amphioxus as from that of the Ascidia--a typical
chordula.


CHAPTER 2.17. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT.

The structural features that distinguish the vertebrates from the
invertebrates are so prominent that there was the greatest difficulty
in the earlier stages of classification in determining the affinity of
these two great groups. When scientists began to speak of the affinity
of the various animal groups in more than a figurative--in a
genealogical--sense, this question came at once to the front, and
seemed to constitute one of the chief obstacles to the carrying-out of
the evolutionary theory. Even earlier, when they had studied the
relations of the chief groups, without any idea of real genealogical
connection, they believed they had found here and there among the
invertebrates points of contact with the vertebrates: some of the
worms, especially, seemed to approach the vertebrates in structure,
such as the marine arrow-worm (Sagitta). But on closer study the
analogies proved untenable. When Darwin gave an impulse to the
construction of a real stem-history of the animal kingdom by his
reform of the theory of evolution, the solution of this problem was