430 HEREDITY AND DEVELOPMENT 



The Material Basis of Inheritance. — It seems that the 

 botanist Nageli was the first to point out that the material basis 

 on which the hereditary tendencies depend must be a minimal 

 quantity of substance. The inheritance from the father and 

 from the mother is potentially equal ; the vehicle of this in- 

 heritance is in the germ-cells ; the mass of a spermatozoon 

 may be only yooVoo^^ "^^xX. of the mass of the ovum which 

 it fertilises ; in one respect the two sex-ceUs are equivalent 

 — they have the same number of stable readily stainable 

 bodies or chromosomes in their respective nuclei ; the number 

 of these bodies is constant for each species, except that the 

 number in the mature sex-cells is half that found in the ordinary 

 cells of the body ; the chromosomes play an obviously important 

 part in the intermingling or amphimixis which occurs in fer- 

 tilisation and in the subsequent divisions of the fertilised egg : 

 for these and other reasons, Weismann concluded in 1885, as 

 Strasburger and O. Hertwig did about the same time, that 

 the hereditary substance is in the chromosomes of the nucleus of 

 the germ-cell. 



Microscopic vivisection experiments on Protozoa — e.g. the 

 trumpet animalcule, Stentor — ^show that a fragment of a cell 

 with a portion of nucleus wiU live on and reconstruct an entire 

 organism, whereas a portion without nucleus, though it lives for 

 a time, is unable to assimilate or recuperate its losses and soon 

 dies. " It is in the nucleus, therefore, that we have to look for 

 the substance which stamps the material of the cell-body with 

 a particular form and organisation — namely, the form and organi- 

 sation of its ancestors/' It goes without saying that the sex-cell 

 is a unity, a minute organism, that its cell-protoplasm (in the 

 case of the ovum at least) represents the building-material 

 (trophoplasm), in which alone the hereditary substance (idio- 

 plasm) can unfold its wonderful powers ; but it must be remem- 

 bered that even a non-nucleated -fragment of an ovum may 

 develop (into a larva at least) if it be fertilised — i.e. supplied 



