MATURATION OF THE GERM-CELLS 45 



6. Maturation of the Germ-cells 



We have seen that the germ-cells owe their capacity of develop- 

 ment to the fact that they are the unspecialised descendants of 

 the parental fertilised ovum the custodians of the characteristic 

 germ-plasm. In some cases the lineage of germ-cells is from 

 the first distinct and apart from the lineage of body-forming 

 cells, and we argue from these clear cases of germinal con- 

 tinuity to the more numerous and less obvious cases where the 

 germ-cells are not recognisable as such until later stages 

 in development. 



There is no need for our present purpose to follow the genera- 

 tions of the germ-cells within the body, or to trace the stages 

 of growth and differentiation between primitive germ-cells 

 and the fully formed ripe ova and spermatozoa. It is 

 necessary, however, to allude to the process of maturation, 

 which has a direct bearing on the problems of heredity and 

 inheritance. 



Maturation. i. It is an elementary fact of histology that 

 the nucleus of each cell in the body of an organism contains a 

 number of readily stainable bodies or chromosomes. In many 

 cases it has been possible to count these, and it has been found 

 that (with a few explicable exceptions) the number is constant 

 for each species. 



As Prof. E. B. Wilson says (1900, p. 67) : " The remarkable 

 fact has now been established with high probability that every 

 species of plant or animal has a fixed and characteristic number 

 of chromosomes, which regularly recurs in the division of all of 

 its cells, and in all forms arising by sexual reproduction the 

 number is even.* Thus, in some of the sharks the number is 36 ; 

 in certain Gasteropods it is 32 ; in the mouse, the salamander, 

 the trout, the lily, 24 ; in the worm Sagitta. 18 ; in the ox, guinea- 



* In a few insects the females have in their body-cells one chromo- 

 some in addition to the number possessed by the males, 



