THEIR SIGNIFICANCE IN HEREDITY. 379 



precisely the same manner in all germ-cells, and finally how in- 

 credible it is that the nuclear thread should always be divided in 

 exactly the same place to form corresponding- loops or rods, we are 

 driven to the conclusion that it is quite impossible for the ' re- 

 ducing- division ' of the nucleus to take place in an identical manner 

 in all the germ-cells of a single ovary, so that the same ancestral 

 germ-plasms would always be removed in the polar bodies. But if 

 one group of ancestral germ-plasms is expelled from one egg, and a 

 different group from another egg, it follows that no two eggs can 

 be exactly alike as regards their contained hereditary tendencies : 

 they must all differ. In many cases the differences will only be 

 slight, that is, when the eggs contain very similar combinations of 

 ancestral germ-plasms. Under other circumstances the differences 

 will be very great, viz. twhen the combinations of ancestral germ- 

 plasms retained in the egg are very different. I might here mention 

 various other considerations ; but this would lead me too far from my 

 subject, into new theories of heredity. I hope to be able at some 

 later period to develope further the theoretical ideas which are 

 merely indicated in the present essay. I only wish to show that the 

 consequences which follow from my theory upon the second division 

 of the egg-nucleus, and the formation of the second polar body, 

 are by no means opposed to the facts of heredity, and even explain 

 them better than has hitherto been possible. 



The fact that the children of the same parents are never entirely 

 identical could hitherto only be rendered intelligible by the vague 

 suggestion that the hereditary tendencies of the grandfather pre- 

 dominate in one, and those of the grandmother in another, while 

 the tendencies of the great-grandfather predominate in a third, 

 and so on. Any further explanation as to why this should happen 

 was entirely wanting. Others even looked for an explanation 

 to the different influences of nutrition, to which it is perfectly 

 true that the egg is subjected in the ovary during its later de- 

 velopment, according to its position and immediate surroundings. 

 I had myself referred to these influences as a partial explanation 1 , 

 before I recognized clearly how extremely feeble and powerless are 

 the influences of nourishment, as compared with hereditary ten- 

 dencies. According to my theory, the differences between the 



1 Weismann, ' Studien zur Descendenztheorie,' ii. p. 306, Leipzig, 1876, translated 

 by Meldola ; see ' Studies in the Theory of Descent,' p. 680. 



