Chapter 24 



EMBRYOLOGIC DEVELOPMENT OF ANIMALS 



Ontogeny; Phylogeny; Recapitulation (Biogenetic) Theory 

 Morphogenesis 



The embryologic stages undergone by an individual in its development 

 from the zygote to the adult are considered as the life history of the in- 

 dividual or ontogeny (on -toj' e ni) (Gr. on, being; genos, develop). A 

 race of organisms, made up of successive generations of individuals also 

 changes and evolves, and this developmental history of a race is known 

 as phylogeny (fi-loj'eni) (Gr. phylon, race; genos, descent). The 

 recapitulation (biogenetic) theory states that the life history of the stages 

 of embryologic development of the individual (ontogeny) briefly recapit- 

 ulates (repeats), in a modified manner, the evolution or stages of devel- 

 opment of the race (phylogeny) . Each organism tends in its individual 

 life history to recapitulate the various stages through which its ancestors 

 have passed in the development and evolution of their particular race. 

 In other words, ontogeny recapitulates phylogeny. 



Each multicellular animal begins its life as a single, fertilized cell which, 

 according to the recapitulation theory, corresponds, in a general way, to 

 the unicellular Protozoa in animal ancestry. Most multicellular animals 

 (few exceptions) pass through an embryologic two-layered stage which 

 is comparable to such two-layered organisms as Hydra, sponges, etc. In 

 the higher multicellular animals there follows an embryologic three- 

 layered stage which is comparable to the three-layered organisms, such 

 as annelids, arthropods, chordates, etc. Many structures are developed 

 in individual organisms which are similar to comparable structures in 

 lower ancestral types of organisms. Embryonic pharyngeal clefts ("gill 

 slits") appear in the developing embryos of all mammals because they 

 developed in their ancestors. In certain adults, such as man, there are 

 no visible pharyngeal clefts remaining, but the embryologic stages of the 

 human being pass through these pharyngeal cleft stages nevertheless. 

 The development of a four-chambered heart in a higher animal em- 

 bryologically recapitulates the types of hearts found in the fishes, am- 



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