8o AGE, GROWTH, AND DEATH 



ence. Nevertheless the constant death of cells is one 

 of the essential factors of development, and much of 

 the progress which our bodies have made during the 

 years we have lived has been conditional upon the 

 death of cells. As we have seen, cytomorphosis, 

 when it goes through to the end, involves not only the 

 differentiation but the degeneration and death of the 

 parts. There are many illustrations of this which I 

 might cite to you as examples of the great importance 

 of the destruction of parts. Thus there is in the em- 

 bryo before any spinal column is formed an easily 

 visible structural axis which is termed the notochord. 

 In the young mammalian embryo this structure is 

 clearly present and plays an important part, but in the 

 adult it has almost disappeared, and its disappearance 

 begins very early during embryonic life. There are 

 numerous blood-vessels which we find to occur in the 

 embyro, both those which carry the blood away from 

 the heart and those which bring blood to the heart, 

 which during the progress of development are entirely 

 destroyed, and disappear for ever. Knowledge of these 

 is to the practical anatomist and surgeon often of 

 great importance. Vast numbers of the smaller blood- 

 vessels which we know commonly by the name of 

 capillaries exist only for a time and are then destroyed. 

 There is in the young frog, while he is in the tadpole 

 stage, a kidney-like organ, which on account of its 

 position is called the head-kidney, but it exists only 

 during the young stage of the tadpole. There is later 

 produced another kidney which, from its position, is 



