1346 PHYSIOLOGY 



cells destined to produce the somatic cells a portion of the chromatin 

 is cast out into the cytoplasm, where it degenerates, so that only in 

 the germ-cells is the sum total of the chromatin retained. Thus in 

 the two-celled stage, in one cell all the chromatin is preserved, while 

 in the other cell the thickened ends of the chromosomes are cast off 

 into the cytoplasm and degenerate, only the thinner central portions 

 being preserved. When these divide again, the same process is 

 repeated in only one of the daughter cells derived from a germ-cell, and 

 the process is repeated during five or six divisions, after which the 

 chromatin elimination ceases, and the two primordial germ-cells 

 thenceforward give rise only to other germ-cells in which the entire 

 chromatin is preserved. Thus " the original nuclear constitution of the 

 fertilised egg is transmitted, as if by a law of primogeniture, only to 

 one daughter cell, and by this again to one, and so on, while in other 

 daughter cells the chromatin in part degenerates, in part is transformed, 

 so that all of the descendants of these side-branches receive small 

 reduced nuclei " (Boveri, quoted by Wilson). 



The immortality, which was the property of all the unicellular 

 ancestors of the metazoa, has in the latter descended only to the germ- 

 cells. All the other cells of the body, which form the nervous and 

 muscular tissues, glands, skin, &c., are mortal. They pass through 

 a certain number of divisions ; but although this number is large, it is 

 limited, and on the number 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 mech- 

 anisms 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 syn- 

 cytium, 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 



