EVOLUTION 



295 



vertebral column is taken to mean evolution from a common 

 ancestral structure, embryogeny becomes a criterion for homol- 

 ogy. The embryonic record in a given case is regarded as an 

 evolutionary record, but one that is often blurred and incomplete, 

 and must therefore be interpreted with great care. The embryos 

 of all vertebrates develop pharyngeal pouches, which in many 

 cases fuse with the ectoderm and break through to form gill 

 clefts. These are visible externally in early human embryos as 



Fig. 173. — Human embryo of about thirty days in age. a, arm bud; c, 

 gill cleft (hyomandibular) ; l, leg bud; m, mesencephalon; mt, metencephalon; 

 my, myelencephalon; o, olfactory pit; T, telencephalon; u, umbilical cord. (After 

 Keibel and Mall, Human Embryology, J. B. Lippincott Company.) 



grooves (Fig. 173), but no actual clefts are formed. Nor are 

 gills developed. Obviously these incomplete gill clefts cannot be 

 compared with the gill clefts of a fish which function as 

 passageways for water, but they may be compared with a corre- 

 sponding stage in the fish embryo. On this basis they may be 

 also compared with the gill clefts of the embryo of the chick, of a 

 reptile, or of an amphibian. The only plausible explanation of 

 gill clefts in the human embryo would seem to be an evolution 

 from an ancester whose embryo possessed gill clefts. In the 

 fish the embryonic gill clefts continue developing, together with 

 gills, to form a functional respiratory organ. In man or the chick 



