FERTILIZATION IN PLANTS AND UNICELLULAE ORGANISMS 339 



quite a specialized physiological function and is histologically adapted 

 to this function, the latter because, from observation on the sperm- 

 atozoon which has made its way into the ovum, we know that it 

 contains the centrosome, the dividing apparatus of the nucleus. Thus 

 there only remains the ' head ' of the spermatozoon, which includes the 

 nucleus, as the possible vehicle of the heritable substance. Therefore 

 we are led to seek for the hereditary substance in the nucleus. But 

 the hereditary substance cannot be a perishable substance which may 

 at need he dissolved, in the literal sense of the word, and be formed 

 anew ; therefore we cannot look for it in the nuclear membrane, 

 and just as little in the ' nuclear sap ' which tills the meshes of the 

 nuclear network, since the material on which heredity dejDends must 

 necessarily be solid. ' Nageli has clearly shown that we must assume 

 a stable, that is, a solid molecular architecture. There thus remains 

 only the nuclear reticulum with its chromatin granules, and when we 

 remember what we have learnt of tlie behaviour of this chromatin 

 substance during division and amphimixis we can entertain no doubt 

 that the sought-for bearer of the inheritance is contained in the sub- 

 stance of the chromosomes. 



The great care with which the chromosomes are halved by means 

 of the complicated division apparatus led us earlier to regard them as 

 a substance of complex and manifold qualities and of great physio- 

 logical importance ; their constant number in any one species, and 

 the reduction of that number to half by means of the reducing 

 divisions, justify us in concluding that they are permanent structures, 

 physiological and morphological units, which undergo no more than an 

 apparent irregular dispersion during the resting state of the nucleus. 

 Finally, the fact that these supposed vehicles of inheritance occur in 

 equal numbers in each of the conjugating germ-cells, and that this 

 number is ahvays, both in animals and in plants, half of the normal 

 number occurring in somatic cells, is decisive. The logical necessity 

 that the hereditary substance of both parents should be transmitted 

 to the offspring in equal quantity could not be more precisely met 

 than it is by the fact that half the normal number of chromosomes 

 occurs in each of the sex-nuclei in the ovum. Personally, I have long 

 been certain, on these grounds, that the chromosomes of the nucleus 

 are the hereditary substance, and I expressed my conviction on this 

 point almost simultaneously with Strasburger and O. Hertwig ^. 



^ More precisely, my conclusions were published several months later than those of 

 the investigators named (1885). I think, however, that no one who is familiar with 

 my writings for the years immediately preceding, which are collected in Avfsatsen uber 

 Vererbimg und verwandte biologische Frageii (Jena, 1892), will dispute that the idea was 

 reached by me independently. I attach importance to this because all my later work 

 is based upon this idea. 



