in the endoderm, while still others, and among them
Haeckel, considered that part of it came from the ectoderm and part from
the endoderm (pp. 23-4, 1874).

The solution of the problem came from those observations on the
development of the lower forms to which we have just alluded.

The early history of these discoveries and of the theory which grew out
of them has been well summarised by Lankester,[446] and may conveniently
be given in his own words:--

"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish,"
in _Contributions to the Natural History of the United States_, vol. v.,
1864) showed in his account of the development of Echinoderma that the
great body-cavity of those animals developed as a pouch-like outgrowth
of the archenteron of the embryo, whilst a second outgrowth gave rise to
their ambulacral system; and in 1869 Metschnikoff (_Mem. de l'Acad.
imperiale des Sciences de St Petersbourg_, series vii., vol. xiv.,
1869), confirmed the observations of Agassiz, and showed that in
Tornaria (the larva of Balanoglossus) a similar formation of
body-cavities by pouch-like outgrowths of the archenteron took place.
Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss.
Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the
relationship of the coelenteric apparatus of the Enterocoela to the
digestive canal and body-cavities of the higher animals. Leuckart had in
1848 maintained that the alimentary canal and the body-cavity of higher
animals were united in one system of cavities in the Enterocoela
(_Verwandschaftsverhaeltnisse der wirbellosen Thiere_, Brunswick, 1848).
Metschnikoff insisted upon such a correspondence when comparing the
Echinoderm larva, with its still continuous enteron and coelom, to a
Ctenophor, with its permanently continuous system of cavities and
canals. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was
formed by a division of the archenteron into three parallel cavities,
and in 1874 demonstrated the same fact for the Brachiopoda. In 1875
(_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to
distinguish three kinds of body-cavity: the schizocoel, formed by the
splitting of the mesoblast, as in the chick's blastoderm; the
enterocoel, formed by pouching of the archenteron, as in Echinoderms,
Sagitta and Brachiopoda; and the epicoel.... Immediately after this I
put forward the theory of the uniformity of origin of the coelom as a