426 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 100 1 000 th part of the mass of the ovum which 

 it fertilises ; in one respect the two sex-cells 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, ^ 

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

 the hereditary substance is in the chromosomes of the nucleus of 

 ihi germ-cell. 



Microscopic vivisection experiments on Protozoa — e.g. the 

 trumpet animalcule, Stentorshow that a fragment of a cell 

 with a portion of nucleus will 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 





