Organs or Ancestors:
Many have traced the demise of Haeckel's theory to the inadequacy of his gastraea—that "lean animal-specter"3—as a satisfactory ancestor for all metazoans; or rather to the general inadequacy—or absurdity—of the almost countless creatures that recapitulationists elevated from their fleeting appearance in modern embryos to hypothetical adult ancestors of great antiquity.
The epitome of Haeckel's Natürliche Schöpfungsgeschichte (1868) is a chart of twenty-two human ancestors based upon embryology, comparative anatomy, and paleontology. Later stages rested heavily on the evidence of fossils and the anatomy of modern animals, but the primeval forms were constructed solely from earliest ontogeny. Though their existence relied upon no other evidence, they were given names and placed at the base of Haeckel's famous tree (Fig. 20). The following account of five stages appears in later editions of Natürliche Schöpfungsgeschichte (1892, for example). Gastraea makes no appearance in the first edition of 1868, since Haeckel did not publish his gastraea theory until 1874.
Stage 1—Monera (the primordial, anucleate ancestors of all animals). 4 Evidence—the monerula (the fertilized ovum after disappearance of the germinal vesicle). As Russell writes: "It was still believed by many that the egg-nucleus disappeared on fertilization. The true nature of the process was not fully made out till 1875, when O. Hertwig observed the fusion of egg- and sperm-nuclei in Toxopneustes, a sea urchin" (1916, p. 291).
Stage 2—Amoeba. Evidence—the cytula (the ovum after reformation of its nucleus). As evidence, Haeckel cited the amoeboid motions observed in some egg cells (Fig. 21). With characteristic assurance, he argued: "An irrefutable proof that such single-celled primeval animals really existed as the direct ancestors of Man, is furnished according to the fundamental law of biogeny by the fact that the human egg is nothing more than a simple cell" (1892, 2:381).
Stage 3—Synamoeba (the first association of undifferentiated amoeboid cells to form the earliest multicellular organism). Evidence— the morula (the mass of cells produced by initial cleavages of the fertilized egg).
Stage 4—Blastaea (an ancestral free-swimming form). Evidence— the blastula (hollow sphere of cells formed after the initial cleavages).
Stage 5—Gastraea (a two-layered differentiated form; the common ancestor of all Metazoa). Evidence—the gastrula (two-layered sac