1204 PHYSIOLOGY 



of divisions which are possible depends the normal duration of life of the 

 organism to which the cells belong. We may thus regard the egg-cell as 

 dividing into two parts. From one part will be formed by differentiation 

 all the complex somatic mechanisms of the adult animal : the other part 

 will divide, but will remain in an undifferentiated form, until its descendants 

 can conjugate with germ- cells from other individuals and form fertilised 

 egg-cells, destined to undergo the same series of changes. 



The metazoan individual thus consists of a mortal host holding within itself the 

 immortal sexual cells or gonads. Gaskell has pointed out that the development of 

 the fertilised ovum involves two parallel processes on the one hand, the elaboration 

 of the elements forming the host ; on the other, of those derived from the free-living 

 independent germ-cells. From the very beginning the somatic part of the organism, 

 the host, is a reacting individual in which the nervous system acts as the integrator of 

 all the activities of the body and as the middleman between the internal and external 

 epithelial surfaces and the muscular system. The host may thus be regarded as a 

 neuro -epithelial syncytium, every step in its evolution and differentiation being attended 

 by increased control of all the units by a central nervous system. 



The gonads were placed at first within the interstices of this syncytium, and escaped 

 to form a new generation only after the death and disintegration of the host. But 

 differentiation and division of labour affect also the free-living gonads. Some of these 

 form a germ epithelium surrounding the body cavity, of which a few only of the elements 

 pass out of the host as perfect germ-cells, while the others are subordinated to the 

 metabolic needs of these germ-cells and are transformed into various elements, such as 

 nurse-cells, wandering mesoderm cells or phagocytes, yolk-cells, and so on. Gaskell 

 regards the greater part, if not the whole, of the connective-tissue framework of the 

 body, as well as the wandering corpuscles of the blood and tissue-fluids, as derived from 

 these primitive germ-cells. All these tissues, though useful to the host as well as to 

 the finally successful germ-cells, present the common feature of an absolute independence 

 of the central nervous system. Thus the evolution of the animal kingdom means 

 essentially the evolution of the host, and must therefore be closely connected with the 

 evolution of the central nervous system, the ruling element in the neuro-muscular 

 syncytium. On these grounds Gaskell has used the comparative morphology of the 

 central nervous system as a means of tracing the origin of the vertebrate from the in- 

 vertebrate type, and has come to the conclusion that the immediate ancestor of the 

 vertebrate must be sought in the invertebrate group presenting the most highly developed 

 central nervous system, namely, the Arthropoda. 



All the complex mechanisms which are concerned in maintaining the 

 life of the individual have apparently been developed in order to give the 

 potentially immortal germ-cells a better chance of survival in the struggle 

 for existence. From the broad biological standpoint, as Foster points out, 

 all the toil and turmoil of human existence may be regarded simply as the 

 by-play of an ovum-bearing organism. From the same standpoint one must 

 acknowledge that the mortality of the individual, resulting from the absence 

 of an indefinite power of multiplication among the somatic cells, must be 

 an advantage to the race. Throughout the living world the welfare of the 

 individual is subordinated to that of the species. With each new genera- 

 tion there are possibilities of variation and of the production of individuals 

 better or worse fitted for the maintenance of the race than those of the 

 previous generation. Immortality of the individual would handicap the 

 survival of the younger generations, and we should have the same retardation 



