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TEXTBOOK OF ZOOLOGY 



was what led Haeckel to formulate tlie recapitulation theory, ex- 

 pressed briefly: ontogeny repeats or recapitulates phytogeny, as has 

 been discussed earlier in the chapter. 



In the vertebrate group these apparent relations are much shown. 

 Most of the embryos of this group are so similar that it is nearly 

 impossible to distinguish them. They pass through the identical 



Fig. 445.- — Diagram to show the modifications of aortic or branchial arches in 

 different vertebrates types. A. primitive scheme; B. hingflsh ; C, primitive am- 

 phibian (urodele) ; D, frog; E, snake; F, lizard; G, bird; H, mammal; ec, ic, 

 external and internal carotids ; va, ventral aorta ; da, dorsal aorta ; db, ductus 

 Botalli ; p, pulmonary artery ; s, subclavian ; c, coeliac. Vessels carrying venous 

 blood are black; tliose with mixed blood are shaded; those which disappear are 

 dotted outlines. (After Boas. Reprinted by permission from Kingsly, Comparative 

 Anatomy of Vertebrates, published by The Blakiston Co.) 



stages of development. Systems, such as the circulatory, nervous, 

 digestive, and respiratory, follow the same course of development 

 in all of the vertebrates, no matter how simple or complex. In 

 earlier stages the similarity is strikingly close. 



