Haeckel's Gastraea theory, though it exercised a great influence upon the subsequent trend of phylogenetic speculation, was by no means universally accepted _telle quelle_. Opinions differed considerably as to the primitive mode of origin of the two-layered sac which was very generally admitted to be of constant occurrence in early embryogeny. Ray Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a "Planula" theory, according to which the ancestral form of the Metazoa was a two-layered closed sac formed typically by delamination, less often by invagination. He denied that the invagination opening (which he named the blastopore) represented the primitive mouth,[443] holding that this was typically formed by an "inruptive" process at the anterior end of the planula, which led to the formation of a "stomodaeum." A similar process at the posterior end gave rise to the anus and the "proctodaeum." The question as to whether delamination or invagination was to be considered the more primitive process was discussed in detail by Balfour,[444] without, however, any very definite conclusion being reached. He held that both processes could be proved in certain cases to be purely secondary or adaptive, and that accordingly there was nothing to show that either of them reproduced the original mode of transition from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by no means rejected the theory that the Gastraea, "however evolved, was a primitive form of the Metazoa," but, having regard to the great variations shown in the relation of the blastopore to mouth and anus (pp. 340-1), he was inclined to think that if the gastrula had any ancestral characters at all, these could only be of the most general kind. Balfour's attitude perhaps best represents the general consensus of opinion with regard to the Gastraea theory. From the same origins as the Gastraea theory arose the theory of the coelom. The term dates back to Haeckel in 1872, and the observations which first led up to the theory were made by the men who supplied the foundations of the Gastraea theory--A. Agassiz, Metschnikoff and Kowalevsky. But it was not Haeckel himself who enunciated the coelom theory. It will be remembered that Remak introduced in 1855 the conception of the mesoderm as an independent layer derived from the endoderm. The pleuro-peritoneal or body-cavity was formed as a split in the "ventral plates" of the mesoderm. Haeckel's "coelom" corresponded to the "pleuro-peritoneal cavity" of Remak, but his view of the origin of the mesoderm brought him much closer to von Baer's conception of the origin of _two_ secondary layers from ectoderm and endoderm respectively than to Remak's conception of the mesoderm as a single independent layer.