EMBRYOLOGY AND EVOLUTION 



form have larvae that are strikingly alike. Zoologists had 

 long classified barnacles as mollusks, until a study of their 

 larvae showed conclusively that they were crustaceans. Many 

 parasitic crustaceans are mere sacs of eggs or spermatozoa 

 in the adult stage, but they have typical crustacean larvae. 

 Ascidians, or sea-squirts, were classified as mollusks until 

 Kowalevsky showed that their larvae are little tadpoles with 

 notochord, dorsal nerve-tube, and gill-slits like any typical 

 vertebrate. These and many' other examples show that nat- 

 ural affinities or ancestral relationships are often shown in 

 embryos and larvae long after they have been obscured or 

 lost in adult stages. 



All vertebrates, from fishes to mammals, pass through a 

 fish-like stage in their development in which they have (1) 

 gill slits, (2) five or six pairs of aortic arches, (3) a simple 

 tubular heart with one auricle and one ventricle, (4) a 

 notochord (the basis of the backbone in higher vertebrates), 

 (5) a primitive type of kidney (the pronephros) , etc. These 

 organs persist throughout life in the lowest fishes, but 

 they undergo many changes in higher forms. 



Entirely similar conditions are found in the development 

 of the brain and central nervous system; the eye and ear; the 

 limbs and muscular system; the digestive, respiratory and 

 reproductive systems. In fact, nearly all the important organs 

 and systems of higher vertebrates pass through stages in 

 their development which in lower vertebrates remain 

 permanently. 



It is true that some of these embryonic reminiscences of 

 lower forms have been modified and greatly abbreviated, but 

 they are nevertheless all there and are recognizable. Among 

 higher forms there are many adaptations to peculiar condi- 

 tions of embryonic or larval life which have no counterpart 

 in lower forms ; such are the embryonic membranes of higher 



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