Recapitulation 81 



siderable specificity even in the early days of develop- 

 ment, and this soon becomes more and more ac- 

 centuated. 



The embryo of a reptile, bird, or mammal always 

 shows a number of gill-clefts on the side of the neck, 

 minute openings from the pharynx to the exterior. 

 They are gill-clefts or visceral clefts. The first seems 

 to become the Eustachian tube, from the outer ear- 

 passage to the back of the mouth, and the thymus 

 gland is developmentally connected with another; 

 but the rest of them are useless and soon pass away. 

 Occasionally in adult life there may be just a trace 

 left, but that is an abnormalitv. Now, there can be 

 no doubt that these gill-clefts are echoes of the past. 

 They are recapitulations of the gill-clefts which are 

 used in the respiration of tadpoles, of fishes, of 

 lampreys, and of still more primitive Vertebrates 

 like lancelets. The evolutionist account of the useless 

 gill-clefts of the higher Vertebrates is the only one 

 that makes sense. They are relics of ancestry, and it 

 is very striking to learn from Dr. Boy den (1918) 

 that in the embryos of the chick and of some reptiles 

 there are not only gill-clefts but dwindling and tran- 

 sient traces of gills. 



In a primitive backboned animal like the lancelet, 

 or even in the much more advanced lamprey, the 

 dorsal supporting axis of the body is called the noto- 

 chord. It is a skeletal rod folded off from the median 

 dorsal line of the embryonic gut ; it therefore belongs 

 to the inner germ-layer or endoderm. In fishes it often 

 persists to a large extent, but it is being more or less 



