68 INTRODUCTION TO EVOLUTION 



the ovum and proceeding in orderly sequence to the adult condition. One 

 event initiates another; the latter initiates a third, and so on. We have ex- 

 perimental proof, for example, that the neural tube (p. 54) will not 

 form unless the particular region of ectoderm from which it is to form has 

 underlying it a notochord. The notochord is an unsegmented rod lying in 

 the position which the backbone will eventually occupy (Fig. 4.12). The 

 notochord appears very early in embryonic development and serves as an 

 organizer, inducing formation of a neural plate and tube in the ectoderm 

 lying over it. Experiments on embryos of lower animals indicate that if the 

 notochord is removed no neural tube will develop. Now a notochord is 

 found in all members of Phylum Chordata. In a few members of the 

 phylum (e.g., Amphioxus, Fig. 5.5, p. 101) it persists throughout life, 

 forming an elastic, stiffening rod down the back. In most members of the 

 phylum, i.e., in vertebrates, it is eventually replaced by a vertebral col- 

 umn, a bony backbone. Thus the presence of a notochord in the embryos 

 of higher vertebrates is an example of recapitulation of a structure present 

 in the embryos of ancestors. Why does the notochord still continue to ap- 

 pear in embryos of these higher vertebrates? Probably because it is needed 

 as an organizer to induce formation of the neural tube. Neural tube 

 formation in the first ancestral chordates probably occurred under the in- 

 fluence of the notochord, and the connection of the latter with neural tube 

 formation was so vital that embryos ever afterward must have noto- 

 chords to induce formation of their neural tubes. The notochord also 

 forms an axis around which the segmental bones comprising the vertebral 

 column develop. It seems likely that the presence of the notochord is also 

 essential to this process of backbone formation, though experimental evi- 

 dence on the point is lacking. 



Again, three kidneys are found in Phylum Chordata. The hagfish (Class 

 Cyclostomata) has the kidney called the pronephros. Other fishes, and 

 the amphibians, have as their kidney the mesonephros. But their em- 

 bryos all develop a pronephros first; then the mesonephros develops and 

 the pronephros disappears. Why does the pronephros develop at all? Al- 

 though we have no direct experimental evidence upon which to base an 

 answer, it seems probable that the pronephros serves as an organizer in- 

 ducing subsequent formation of the mesonephros. One evident contribu- 

 tion is made by the pronephros to the mesonephros: the duct leading from 

 the pronephros to the exterior is taken over by the mesonephros for the 

 conveying of its waste products. In reptiles, birds, and mammals the kid- 

 ney is the metanephros. But again the embryos faithfully recapitulate an- 

 cestral embryonic history, developing first a pronephros (which never 



