364 THE CHANGING GENERATIONS 



the embryos of reptiles, birds, and mammals have still longer series of embryonic 

 changes to go through; they have to proceed well past the amphibian level of 

 organization. 



Whatever assemblage of embryos we examine, we find in general that the 

 characteristics that differentiate the classes appear before those which separate 

 the orders, those of the orders before those of the families, etc. Despite occasional 

 exceptions, the differences between closely similar species are the last to appear. 



The course of development may be compared to a journey that each 

 individual must make from zygote to adult. At the beginning we have a 

 broad highway, along which moves a vast throng of individuals traveling 

 in company. Small groups soon begin to turn aside into lesser roads; but 



Fig. 24.7. Corresponding stages in the embryonic development of fish, salamander, reptile, 

 bird, and mammal, showing the greater similarity that exists between the embryos than 

 between the adults. (Courtesy Chicago Natural History Museum.) 



most of the travelers keep on together until the highway begins to send 

 off main branches toward different provinces. These provinces and their 

 subdivisions are the phyla, classes, and orders. The body of travelers 

 splits up, each group following the course that leads toward its home. 

 The roads divide into tracks, the tracks into paths, and each path leads 

 to a single village, whose inhabitants are all members of a single species. 

 Those who reach this village have for a time traveled in company with 

 others bound for distant provinces; but only the members of this particu- 

 lar species have followed the same developmental path to the end. 



We may summarize the findings of comparative embryology as follows: 



1. Simpler organisms undergo fewer embryonic changes to reach the 

 adult condition than do more complex "higher" organisms. 



2. Among the members of a given phylum the sequence of changes 

 undergone by the embryo is (with few exceptions) invariable. 



