CHAPTER 



EVOLUTION AS SEEN IN 



EMBRYONIC DEVELOPMENT 



Homology in Embryos 



In the preceding chapter we saw that similarities of 

 adult structure not connected with similar habits and adaptations are most 

 reasonably explained as the result of inheritance from common ancestry. 

 In the present chapter we shall consider similarities existing among em- 

 bryos. 



It is a striking fact that there are not only many evidences of common 

 patterns in the adult structures of diverse animals but evidences of com- 

 mon patterns in embryonic development. Indeed, the two phenomena are 

 related, since embryonic development is the process by which adult struc- 

 ture is attained. We might anticipate, therefore, that similar final results 

 would usually be achieved by similar developmental processes. 



Some of these embryonic similarities are displayed in Fig. 4.1, which 

 represents six stages in the embryonic development of six diff"erent 

 animals, ranging from fish to man. Each sequence begins with a single cell, 

 the fertilized egg or ovum, shown at the bottom of each of the six verti- 

 cal columns. To facilitate comparison the ova are all drawn about the 

 same size, although there are actually large size dift'erences. Thus, the hu- 

 man ovum measures only about 1/250 of an inch in diameter while the 

 ovum of a shark measures in the neighborhood of 2 inches. Each is a sin- 

 gle cell, however, containing genetic contributions from both mother and 

 father. Size dift'erences depend mainly upon the amounts of food mate- 

 rial — yolk — present. In the fish egg enough yolk must be provided to 

 nourish the embryo until it is sufficiently developed to begin actively secur- 



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