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 aUude 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. 



