EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 



dorsal tubular nerve cord 



75 



pharyngeal gill slits 



FIG. 4.17. Internal anatomy of a larval tunicate {Molgula). Diagrammatic. (From Buchs- 

 baum. Animals Without Backbones, 2nd ed., University of Chicago Press, 1948, p. 318.) 



creature relatively late in its evolutionary history. The mode of life is an 

 adaptation enabling it to occupy a particular niche in the economy of 

 marine life. Adult Molgula is sometimes referred to as "degenerate." Since 

 it seems, however, to be a successful animal, adequate to the require- 

 ments of life as it finds them, such anthropocentric aspersions are of doubt- 

 ful wisdom. 



Embryonic Vestiges 



In the preceding chapter we called attention to rudimentary or vestigial 

 structures (pp. 40-44). We found their presence in adult animals most 

 reasonably explained as due to retention of reduced remnants of organs 

 functional in ancestors. Embryos frequently commence the development of 

 organs which will not appear in the adult. Such abortive beginnings may 

 be thought of as embryonic rudiments or vestiges. 



The tail of the human embryo mentioned above (p. 65) clearly falls in 

 this category. Whalebone whales (Fig. 3.3, p. 24) do not have teeth in 

 their jaws, yet in some species the tooth germs appear in the embryo, only 

 to disappear. Insects never have paired appendages (pp. 37-39) at- 

 tached to the body segments comprising the abdomen. Yet in the em- 

 bryos of some beetles the beginnings of such appendages appear, to disap- 

 pear later. The forelimbs of whales have been developed into flippers, 

 while the hind limbs have been lost (except for vestiges of bone buried in 

 the flesh. Fig. 3.3). Yet whale embryos develop fore- and hind-limb buds, 

 as does the human embryo (Fig. 4.10); the hind-limb buds later degen- 

 erate. Sheep, in common with cows, deer, antelopes, and their kin, have no 

 clavicle or "coflarbone." Yet the beginnings of a clavicle appear in the 

 sheep embryo and disappear later. Other examples might be given. The 

 occurrence of such embryonic rudiments seems most reasonably explained 

 as resulting from recapitulation of ancestral characteristics. 



