40 BIOLOGY OF DEATH 



extensive literature on the Keimbahn one should consult 

 the recent contributions of Hegner on the subject. 



To condense a long and complicated matter we may 

 state the situation regarding reproduction and death in 

 the Metazoa in this way. A higher, multicellular indivi- 

 dual may be conceived, from the viewpoint of the present 

 discussion, as composed of two essentially independent 

 portions : the germ cells on the one hand, which are im- 

 mortal in the same sense that the Protozoa are immortal, 

 and the rest of the body, which it is convenient to call 

 technically the soma, on the other hand. The soma under- 

 goes natural death after an interval of time which, as we 

 have seen, varies from species to species. The germ cells 

 which the individual bears in its body at the time of its 

 death of course die also. But this is purely accidental 

 death so far as concerns the germ cells. Such of them as 

 were, prior to the death of the soma, enabled to unite 

 with other germ cells went on living just as does the 

 dividing Paramecium. Reduced to a formula we may say 

 that the fertilized ovum (united germ cells) produces a 

 soma, and more germ cells. The soma eventually dies. 

 Some of the germ cells, prior to that event, produce 

 somata and germ cells, and so on in a continuous cycle 

 which has never yet ended since the appearance of multi- 

 cellular organisms on the earth. 



The contrast between the protozoan and the metazoan 

 method of descent is shown in Figure 11, which is a modi- 

 fication of a similar diagram originally due to my col- 

 league, Dr. H. S. Jennings. 



The diagram represents the descent of generations. 

 The upper portion of the diagram shows the mode of 

 descent in forms reproducing from organisms reproduc- 

 ing from a single parent. The lower, or B portion of the 



