), (7), xvi.,
1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871.

[415] "The Old Mouth and the New," _Anat. Anz._, iii.,
1888. _Nature_, xxxix., 1889.

[416] "Recherches sur la Morphologie des Tuniciers,"
_Arch. de Biol._, vi., 1887.

[417] "Die Stellung u. Bedeutung der Morphologie," _Morph.
Jahrb._, i., pp. 1-19, 1876.

[418] "Anatomie des Balanoglossus," _Mem. Acad. Sci. St
Petersbourg_ (Petrograd), (7), x., 1866.

[419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view
of the relation of the Enteropneusta to the Echinoderma,
see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94,
1914.

[420] In a series of papers published in 1884-6, the
speculative results being discussed in his memoir on
"The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi.,
pp. 535-71, 1886.

[421] Reprinted in _Zoological Articles_, London, 1891.

[422] "Die Enteropneusten des Golfes von Neapel," _Fauna
und Flora des Golfes von Neapel_, Monog. xviii., Berlin,
1893.

[423] See Macbride, "A Review of Prof. Spengel's Monograph
on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The
Early Development of Amphioxus," _Q.J.M.S._, xl., 1898.

CHAPTER XVI

THE GERM-LAYERS AND EVOLUTION

In his papers of 1866 and 1867 Kowalevsky had remarked upon the
widespread occurrence of a certain type or fundamental plan of early
embryonic development, characterised by the formation, through
invagination, of a two-layered sac, whose cavity became the alimentary
canal. This developmental archetype was manifested in, for instance,
_Sagitta_,[424] _Rana_,[425] _Lymnaea_,[426] _Astacus_,[427]
_Phoronis_,[428] _Asterias_,[429] _Ascidia_,[428] the _Ctenophora_,[428]
and _Amphioxus_.[428] He noticed also that the invagination-opening
often became the definitive anus. Further instances of this mode of
development were later observed by Metschnikoff[430] and by
Kowalevsky[431] himself, but it was left to Haeckel to generalise these
observations and build up from them his famous Gastraea theory. This was
first enunciated in his monograph of the calcareous sponges,[432] and
worked out in detail in a series of papers published in 1874-76.[433]

Haeckel maintained that the "gastrula" stage occurred in the development
of all Metazoa, and that it was typically formed, by invagination, from
a hollow sphere of cells or "blastula." This typical formatio