INTRODUCTION 2^ 



many excellent observers on the process of fertilisation have 

 now rendered it almost certain — in my opinion, absolutely so 

 — that by far the larger part of the egg-cell does not consist 

 of hereditary substance, and that the latter only constitutes a 

 small portion even of the sperm-cell. From his observations on 

 the egg of the star-fish, Oscar Hertwig had suspected that the 

 essential part of the process of fertilisation consists in the union 

 of the luiclei of the egg- and sperm-cells, and as it is now known 

 that the hereditary substance is undoubtedly contained in the 

 nucleus, this view has, in this respect at least, proved to be 

 the right one. It is true that the nucleus of the male cell is 

 always surrounded by a cell-body, and that Strasburger's opinion 

 to the contrary is incorrect. We now know, through the 

 researches of Guignard, that even in Phanerogams a small cell- 

 body surrounds the nucleus, and that a special structure, the 

 '- centrosome,' — which is absolutely essential for the commence- 

 ment of development, — is contained within it. This structure 

 will be treated of in further detail presently, but I must here lay 

 stress upon my view, that the '• centrosotne^ with its 'sphere of 

 attraction ' cannot in any case be the hereditary siibstatice, and 

 that it is jnerely an apparatus for the division of the cell and 

 nucleus. 



Both in animals and plants, however, essentially the same 

 substance is contained in the nucleus both of the sperm-cell and 

 egg-cell : — this is the hereditary siibstance of the species. There 

 can now be no longer any doubt that the view which has been 

 held for years by Strasburger and myself is the correct one, 

 according to which the nuclei of the male and those of the female 

 germ-cells are essentially similar, i.e.., in any given species they 

 contain the same specific hereditary substance. 



The splendid and important investigations carried out by 

 Auerbach, Biitschli, Flemming, and many others, on the detailed 

 processes of nuclear division in general, and those dealing more 

 particularly with the fertilisation of the egg in Ascaris by van 

 Beneden, Boveri, and others, have given us the means of ascer- 

 taining more definitely what portion of the nucleus is the 

 substance on which heredity depends. As already remarked, 

 this substance corresponds to the ' chromosomes,^ those rod-like, 

 looped, or granular bodies which are contained in the nucleus, 

 and which become deeply stained by colouring matters. 



As soon as it had been undoubtedly proved that the nucleus^ 



