236 PRINCIPLES OF ANIMAL BIOLOGY 



cells to the somatic cells that is very useful in the study of genetics and 

 evolution. Although an anatomical examination of the body of one of 

 the metazoa would indicate merely that the gonad (ovary or testis) is an 

 organ among a host of other organs, there are reasons for not regarding 

 these organs as being of equal rank. Referring only to the germ cells 

 themselves, and not to the surrounding connective tissue or epithelia whose 

 cells can never have reproductive functions, the gonads must be thought of 

 as occupying a position of superiority. The body is not merely a collec- 

 tion of organs, all of which have come from a common source, the 

 fertilized egg. The fertilized egg is rather a detached representative of 

 the groups of germ cells of the parents, from which a soma (body) has 

 been produced. Viewed in this light, the germ cells are a reserve supply 

 out of which in each generation a soma is erected. 



Immortality of Germ Cells. — Germ cells possess a potential immor- 

 tality. That is, each oogonium or spermatogonium has a chance of pro- 

 ducing other germ cells through an indefinite number of cell divisions. 

 There is no necessity of death for any particular germ cell, nor for any 

 specific one of its descendants. It is true, germ cells do die in great 

 numbers. Many eggs are never fertilized, and perish. Eggs and 

 spermatozoa are more often than not lil^erated at times when they have 

 no chance of surviving in new individuals. Gonads always contain 

 some germ cells when the body in which they are located dies, or when 

 the reproductive function of the individual ceases. These remaining 

 cells of course perish. But in each of these cases there is nothing inherent 

 in the germ cells as germ cells which determines that particular cells 

 shall be lost. Each cell has the capacity for escaping from association 

 with its fellows, a chance of uniting with another germ cell in fertilization, 

 an opportunity of giving rise to a new individual in which descendants 

 of the germ cell shall live on and have a chance of surviving. Although 

 some germ cells are inevitably lost each individual cell has the possibility 

 of avoiding that fate and of being permanent in the form of germ cells 

 descendent from it. The germ cells are potentialh^ immortal. 



Contrasted with the possible permanence of germ cells is the neces- 

 sary mortality of the somatic cells. No metazoan body is capable of 

 endless existence. Body cells or their descendants may live on in unin- 

 terrupted series for hundreds of years in some animals, but they eventually 

 die. No final physiological reason can be assigned for the necessity of 

 death, but experience indicates that there is no escape from it. While 

 the individual is said to have descendants, it is only through the germ 

 cells that these are made possible. Strictly speaking, it is only the germ 

 cells that have descendants, and the somatic cells of the next generation 

 are some of those descendants. 



Germ Cells a Reserve Supply. — The germ cells may in a sense be 

 regarded as being the animal. They constitute a series that may be 



