HEREDITY 491 



in connection with the gill-clefts have been recently discov- 

 ered (Boyden, 1918). 



Similarly, the embryos of higher Vertebrates show for a 

 time a notochord, a primitive skeletal axis derived from 

 the roof of the embryonic gut, and thus of endodermic origin. 

 It persists throughout life in lancelets and lampreys, serving 

 as the dorsal axis of the animal, as the forerunner of the 

 backbone which, from fishes onwards, develops from the 

 mesodcrmic sheath of the notochord. The notochord does 

 not become the backbone, though perhaps serving as a sort 

 of tissue-scaffolding for it, and every stage of the replace- 

 ment of the notochord by its substitute the backbone is seen 

 in fishes. Yet on to man himself the notochord continues to 

 appear in development, a veritable antique; it has its short 

 day and passes, leaving but an unimportant trace behind. 



In the establishment of the brain, the skull, the heart, the 

 kidneys, and other important structures in higher animals, 

 the foundations are laid down on old-fashioned lines, not 

 directly suggestive of what is to follow. In the individual 

 organogenesis there is often a recapitulation of historical 

 stages. The development of many an organ appears to the 

 observer to be circuitous, as if the old paths had to some 

 extent to be retrod, and yet the progress of a hundred thou- 

 sand years may be condensed into one day. 



Another aspect of the same fact is that the developing 

 embryos of, say, bird and reptile are for some days very 

 much alike, moving on parallel lines along the great high- 

 way of Amniote development ; but, sooner or later, about 

 the sixth day in the case of the chick, their paths diverge 

 and become distinctively avian and saurian. Thus does the 

 past live again in the present with compelling force. How 

 are we to think of it? 



