486 CONCLUSIONS 



pouches of mammalian embryos resemble ; indeed, not much 

 observation is needed to see that between the gill-pouches of the 

 mammalian embryo and the gill-slits of an adult fish there is but 

 little resemblance, whereas the gill-pouches of embryonic stages 

 are very similar in all groups of vertebrates. This explanation 

 covers all cases of so-called recapitulation. It follows that it 

 is inaccurate and misleading to say that Ontogeny (the develop- 

 ment of the individual) recapitulates Phylogeny (the evolution 

 of the race). What may be true is that Ontogeny recapitulates 

 the Ontogeny of the ancestor, and even then, it is not necessarily 

 true of all embryonic forms. While the gill-pouches do 

 recapitulate in this sense, other organs such as the primitive 

 streak or the extra- embryonic ccelom do not. It is also to be 

 noted that the order of appearance of structures in Ontogeny 

 is not necessarily the same as in Phylogeny. Denticles 

 appeared early in evolution, bat they arise late in the develop- 

 ment of the dogfish. The embryo is phylogenetically older 

 than the amnion, but in the development of the mouse, the 

 amnion arises first and the embryo afterwards. 



The real value of embryology from the point of view of 

 evolution lies in the fact that embryonic forms are like the 

 embryonic forms of related animals. As a rule, the younger 

 the embryos are, and the closer akin the species to which 

 they belong, the more closely do the embryos resemble one 

 another. The more closely allied the species are, the longer 

 does the resemblance between the embryos persist. Embry- 

 ology furnishes valuable evidence therefore as to affinities, 

 but it cannot profess to give definite information concerning 

 the adult forms of ancestors. 



Literature 



Bateson, W. Problems of Genetics. Yale University Press, 1913. 

 Garstang, W. The Theory of Recapitulation : a Critical Restatement of 



the Biogenetic Law. Journal Linnean Society, London, Zoology, vol. 



35> 1922. 

 Goodrich, E. S. Metameric Segmentation and Homology. Quarterly 



Journal of Microscopical Science, vol. 59, 191 3. 

 Huxley, J. S. Constant Differential Growth-ratios and their Significance. 



Nature, vol. 114, December 20th, 1924. 



Versluys, J. Uber die Riickbildung der Kiemenbogen bei den Selachii. 

 Bijdragen tot de Dierkunde, vol. 22, 1922. 



