[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] Lymnæa,[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 Gastræa 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 formation might be masked by cenogenetic modifications caused chiefly by the presence of yolk. The gastrula stage was the palingenetic repetition of the ancestral form of all Metazoa, the Gastræa.