300 LECTURES ON BIOLOGY 



the egg-nucleus, as well as the sperm-nucleus, each contain the 

 ' primary constituents ' of a complete organism. The fertilized 

 egg-cell contains, therefore, within itself potentially the ' rudi- 

 ments ' or ' primary constituents ' of two individuals, of which 

 one possesses the paternal, the other the maternal characters. 

 The growing embryo has, therefore, at its disposal each of its 

 characters in duplicate, a fact which, as we shall see later, 

 has an important bearing upon the doctrine of heredity. 



While individuals that were produced by artificial partheno- 

 genesis possess in their body-cells only one half of the chromo- 

 somes that are possessed by those which originate sexually, we 

 find in those animals that were produced by natural partheno- 

 genesis always the normal number of chromosomes. According 

 to numerous investigators this arises from the fact that in the 

 maturing of the egg-cells only one * directive-body ' is formed, 

 but that the second division, which, as we know, conditions the 

 reduction of the number in the chromosomes, is omitted. In 

 some cases formation of a second ' directive-body ' can take 

 place even in ova which develop parthenogenetically, but it 

 remains in the ovum and afterwards fuses again with the egg- 

 nucleus, so that here, too, we find the normal number of nuclear 

 loops. 



This circumstantial and apparently purposeless procedure 

 may perhaps be explained by assuming that the relevant animals, 

 speaking phylogenetically, have reached the stage of partheno- 

 genetic reproduction since comparatively recent time ; they 

 prepare, therefore, their ova for fertilization atavistically and 

 ' cancel ' these preparations only at the last moment. 



We are now sufficiently prepared for turning our attention to 

 the theory of heredity. When we have freed this problem of all 

 mystic accretions it resolves itself into this question, How is it 

 that the children resemble their parents? Each single higher 

 organism consists of an enormous number of different cells and 

 cell-products which join together to form tissues and organs, and 

 perform all life-functions. Only one cell-species, the germ-cells, 

 have no share in the labours and cares for the well-being of the 

 individual, for theirs is the higher duty of caring for the 

 preservation of the species. We see that again and again there 

 proceed from these germ-cells all the different kinds of cell- 



