SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 361 



sometimes they must be removed surgically. The vermiform appendix is notorious 

 as ;i source of danger to its possessor. It is homologous with a large intestinal 

 pouch or caecum that plays an important part in digestion among many herbi- 

 vorous mammals. The coccyx is nothing more than a short, buried vestigial tail. 

 The pineal body in the brain, although it may have acquired new functions, is 

 morphologically a buried vestige of a third, median eye that was functional in 

 the ancestral land vertebrates and persists in its original form in the lizardlike 

 tuatera, a "living fossil." 



The significance of vestigial structures is plain. They are homologous 

 with structures that were fully developed in the ancestors of the species 

 that possess them. Often we find their homologues still functional in 

 related species living today. Organisms have rudimentary structures 

 because they inherited the genes that produce them; the vestigial condi- 

 tion of these structures is the result of changes that have occurred in the 

 gene complex. The presence of such organs demonstrates the kinship 

 of their possessors to other species, contemporary or ancestral, in which 

 the organs are or were fully functional. 



Embryonic homologies. When we compare the developmental stages 

 of organisms belonging to different species, we find (1) that embryos of 

 related species resemble one another more closely than do the adults of those 

 species and (2) that embryos of the "higher" groups resemble adults of the 

 "lower" groups. These phenomena are most clearly shown in animals, 

 although the same generalizations apply among plants. We may illustrate 

 them by describing an imaginary experiment. 



Suppose we begin with a set of living zygotes, or fertilized eggs — one for each 

 of the thousands of species of animals now living. Examining these zygotes, we 

 find that although they differ in size and in the amount and distribution of yolk, 

 all are single cells, and none shows any indication of the kind of animal it will be 

 as an adult. Now we give the signal for development to start. Cell division begins 

 at once. In some cases this is all that happens; the cells separate to take up life 

 as single-celled organisms, which reproduce asexually until the time comes for 

 conjugation and the production of new zygotes. These are the single-celled 

 protozoans. Most of the zj'gotes, however, undergo rapid segmentation into 

 smaller and smaller cells that stick together. The result is the production of a 

 blastula (Fig. 15.13), which typically is a hollow ball of cells with its wall one 

 cell-layer thick. A few colonial Protista, such as Volvox, drop out of the race at 

 this point. As mature free-living colonies they correspond structurally to the 

 blastula stage of higher animals. 



Nearly all the remaining blastulae proceed to infold or invaginate and are thus 

 converted into gastrulae. In its typical form the gastrula (Fig. 15.14) is cup- 

 shaped, with a central cavity (archenteron) , a single opening (blastopore), and 

 a wall two cell-layers thick (ectoderm and endoderm). The blastulae of sponges 

 do not become true gastrulae but undergo a different type of infolding and soon 

 transform into two-layered adult sponges. Some of the true gastrulae also soon 



