324 LECTURES ON BIOLOGY 



be able to claim a heuristic value. I am following here again 

 essentially the arguments of Gross, because I regard his attempt 

 as comprehensive and intelligible, but disagree with him when 

 he, in accordance with Weismann, conceives the single nuclear 

 loops to be composed of several equivalent ids. I rather believe 

 the simple hypothesis that the different ' primary constituents ; 

 are capable of independent reproduction to be an entirely satis- 

 factory explanation. It is also far better supported by the facts of 

 observation. 



The fundamental condition of Mendelian ' splitting,' the 

 purity of the germ-cells of the hybrids, is, as we have seen, 

 guaranteed by the process that takes place during the maturing 

 of the ovum and sperm, and it only remains now to explain why, 

 in spite of this purity of the sexual products, most crosses produce 

 mixed forms, and why not all ' character-pairs ' behave alike. 



Our observations compelled us to assume that each cell of the 

 body of an organism originated bi-sexually contains the complete 

 hereditary substance, as it were, in two ' editions/ from the father 

 and from the mother. While during the nucleus-division the 

 idioplasm becomes visible in the shape of individualized chromo- 

 somes, it lies in the meantime during the so-called nucleus- 

 rest distributed throughout the entire interior of the nucleus. 

 Only during the preparation for a new reproduction the 

 chromatin fragments collect and arrange themselves once more 

 into the characteristic nuclear rods or loops. We may now 

 contend with some degree of certainty that the chromosomes 

 are not uniform formations, but that they consist of an enormous 

 mass of different 'primary constituents' (about the nature of 

 which we know nothing) of which each corresponds to a certain 

 body-part. If the two parents from which each half of the chromo- 

 some complement came resembled each other closely the conclu- 

 sion is not far-fetched that the paternal and maternal rudiments 

 resemble each other; indeed, according to Gross, they are so 

 much alike that they may be arbitrarily exchanged. They are of 

 course always corresponding ' rudiments,' as otherwise the young 

 would exhibit most remarkable defects. When therefore the 

 nucleus awakens from the rest-stage into activity the different 

 pairs of chromosomes reassume once more their definite number 

 and form, but they are no longer the same chromosomes as 

 heretofore. They are no longer purely paternal or purely 



