76 



Reversions 



INTRODUCTION TO EVOLUTION 



In addition to the normally occurring embryonic rudiments embryos oc- 

 casionally develop abnormal structures reminiscent of normal structures 

 possessed by other, usually "lower," animals. Such abnormal structures are 

 called reversions or atavisms. Occasionally, for example, a human baby 

 is born with a short, fleshy tail protruding from the base of the spine. Since 

 the vertebral column does not extend into the atavistic tail it is easily re- 

 moved by a surgeon. Tails as long as 8 inches have been recorded. 



We have mentioned the pharyngeal pouches, together with the fact that 



only one of them actually perforates to 

 form an opening from the pharynx to the 

 exterior (Eustachian tube plus external 



^ 1^^ /-(«^^^^cM^ ^^^ canal, pp. 61-62). Occasionally 

 ^ ' '■ ' ^' an additional one of these pouches will 



form an opening to the exterior. The re- 

 sult is a cervical fistula, an opening from 

 the nasal cavity or throat to the surface 

 of the head below the ear or to the surface 

 of the neck, the exact position depending 

 upon which of the pouches forms the 

 fistula (Fig. 4.18). It will be recalled that 

 in fishes all of the pouches open to the 

 exterior in this manner to form the gill 

 slits through which water passes in the 

 process of respiration (Fig. 4.11). A 

 cervical fistula, then, results from return 

 by one pharyngeal pouch to embryonic 

 procedures normal in a fish embryo. 

 Human beings, in common with other primates, are provided with one 

 pair of mammary glands located in a pectoral position (on the chest). 

 Many lower mammals, pigs and dogs being familiar examples, have a row 

 of mammary glands extending along both sides of the chest and abdomen. 

 Occasionally human beings are born with extra mammary glands or extra 

 nipples, reminiscent of the condition normal in lower mammals. 



One most interesting example of reversion is found in the horse. In a 

 later chapter we shall note that some of the prehistoric horses regarded as 

 ancestral to the modern horse had three toes on each foot (Fig. 10.5, 

 p. 202). The third digit was enlarged to form a hoof but the second and 

 fourth digits were also present, though of smaller size. We have already re- 



FIG. 4.18. Cervical fistula originat- 

 ing from the second pharyngeal 

 pouch. (From Arey, Developmental 

 Anafomy, W. B. Saunders Co., 

 1947, p. 179.) 



