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 "gastraea." 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 von Baer, and was clearly stated by 

 Fritz Miiller in 1863, but Haeckel first gave it broad application. For 

 a time embryologists, in an excess of zeal for discovering instances of 



