58 



INTRODUCTION TO EVOLUTION 



Evidently a segmental arrangement of body-wall musculature is an 

 efficient mechanism for rapid swimming. Fish embryos in process of de- 

 veloping such a mechanism start with rows of somites. But why do all 

 other vertebrate embryos also follow this pattern regardless of whether or 

 not the adults into which they develop are ever going near the water? The 



^olk SOIG 



amnion 



brain 



FIG. 4.8. Human embryo of about twenty days. Amnion partially cut away to 

 reveal embryo. (After Corner, in Contributions to Embryology, Carnegie Institution 

 of Washington, Vol. 20, 1 929.) 



most reasonable answer seems to be that this is an embryonic pattern in- 

 herited from aquatic ancestors, i.e., from fishes. We shall see that this is 

 only one of many reasons for thinking that fishes are the ancestors, near 

 or remote, of all vertebrates living on land. 



While the neural tube and rows of somites have been forming in the hu- 

 man embryo (Fig. 4.8) other changes have also been occurring. The heart 

 has started to form, for example (Fig. 4.9). As the body increases in size 

 the embryo "bulges up" more and more into the cavity above the em- 

 bryonic disc — the amniotic cavity. Soon this cavity would be filled but for 

 the fact that it increases in size with enlargement of the embryo. The cav- 

 ity contains a fluid, the amniotic fluid, surrounding the embryo and pro- 

 tecting it from injury (Fig. 9.12, p. 191). Soon the embryo is free from 



