THE RISE OF EMBRYOLOGY 71 



must go backwards and follow the embryological 

 record to see where the modifications of structure 

 occurred. 



The most unexpected and most illuminating expe- 

 riences come from this source. In the embryos of 

 higher animals are found many traces of former 

 ancestral conditions. Evanescent organs as gill- 

 slits (with appropriate circulation) occur in all higher 

 animals including the human body. The gill-slits of 

 birds and mammals are transient and never come to 

 functional use. But their presence is indubitable, 

 and we begin to realize that they are clues to the 

 structural features of ancestral forms. They have 

 been handed along by heredity and although not 

 rising to a functional condition, they exist as hered- 

 itary survivals from the days when the ancestors of 

 birds and mammals were aquatic and had use for 

 gill-slits in respiration. The presence of rudimentary 

 teeth hi the jaws of the embryos of toothless whales 

 have the same significance. 



Besides the transitory hereditary survivals, there 

 are many rudimentary organs such as rudimentary 

 ear muscles, tail muscles and the vermiform appendix 

 of the human body which are interpreted in a similar 

 way. 



