. The chordates. Chordata. History of Comparative Anatomy 349. Fig. 283 (Left). Karl Gegenbaur (1826-1903). (Courtesy, Locy: "Biology and Its Makers," New York, Henry Holt & Co., Inc.) Fig. 284 (Right). Ernst Haeckel (1834-1919). (Courtesy, Locy: "Biology and Its Makers," New York, Henry Holt & Co., Inc.) It was at this period that the theories of Ernst Haeckel (Germany: 1834-1919) came into prominence. He was impressed by the fact that metazoan animals pass through an early stage in which the embryo consists of two layers of cells, ectoderm and endoderm, enclosing
. The chordates. Chordata. History of Comparative Anatomy 349. Fig. 283 (Left). Karl Gegenbaur (1826-1903). (Courtesy, Locy: "Biology and Its Makers," New York, Henry Holt & Co., Inc.) Fig. 284 (Right). Ernst Haeckel (1834-1919). (Courtesy, Locy: "Biology and Its Makers," New York, Henry Holt & Co., Inc.) It was at this period that the theories of Ernst Haeckel (Germany: 1834-1919) came into prominence. He was impressed by the fact that metazoan animals pass through an early stage in which the embryo consists of two layers of cells, ectoderm and endoderm, enclosing a cavity which is the prospective digestive cavity, opening to the ex- terior by way of a single aperture, the blastopore (Fig. 285). This gas- trula form, modified in one way or another, occurs almost universally in metazoan embryos. He interpreted it as signifying common ancestry of all metazoans. He created a purely hypothetic common ancestor to which he gave the name "; In form and structure the "gas- traea" was essentially like a gastrula and also similar to a simple two- layered animal (coelenterate) such as Hydra (Fig. 285). He pointed out also that the more highly differentiated metazoans not only pass through the supposedly ancestral gastraea stage but later produce various briefly transitory structures whose obvious counterparts in embryos of "lower" or presumably ancestral animals persist to become functional organs of the adult. The temporary presence of pharyngeal clefts and a notochord in embryos of reptiles, birds, and mammals is a striking example of this. On the basis of such facts, he elaborated the theory of "Recapitulation" or "Law of Biogenesis," which as- serted that ontogeny (the embryonic development of the individual) repeats or "recapitulates" phylogeny (the evolutionary development of the race). The essential idea in the theory had been recognized by earlier embryologists, especially
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