THEIR SIGNIFICANCE IN HEREDITY. 359 



the nucleus are distributed to the daughter-nuclei in such a way 

 that each of them receives only half the number contained in the 

 original nucleus. After Roux's 1 elaborate review of the whole 

 subject, we need no longer doubt that the complex method of 

 nuclear division, hitherto known as karyokinesis, must be con- 

 sidered not merely as a means for the division of the total quantity 

 of nuclear substance, but also for producing a division of the 

 quantity and quality of each of its single elements. In by far the 

 greater number of instances the object of this division is obviously 

 to effect an equal distribution of nuclear substance in the two 

 daughter-nuclei, so that each of the different qualities contained in 

 the mother-nucleus is transferred to the two daughter-nuclei. This 

 interpretation of ordinary karyokinesis is less uncertain than per- 

 haps at first sight it may appear to be. We cannot, it is true, 

 directly see the ancestral germ-plasms, nor do we even know the 

 parts of the nucleus which are to be looked upon as constituting 

 ancestral germ -plasm ; but if Flemming's original discovery of the 

 longitudinal division of the loops lying in the equatorial plane of 

 the nuclear spindle is to have any meaning at all, its object must 

 be to divide and distribute the different kinds of the minutest 

 elements of the nuclear thread as equally as possible. It has been 

 ascertained that the two halves produced by the longitudinal split- 

 ting of each loop never pass into the same daughter-nucleus, but 

 always in opposite directions. The essential point cannot therefore 

 be the division of the nucleus into absolutely equal quantities, but it 

 must be the distribution of the different qualities of the nuclear 

 thread, without exception, in both daughter-nuclei. But these dif- 

 ferent qualities are what I have called the ancestral germ-plasms, i.e. 

 the germ-plasms of the different ancestors, which must be contained 

 in vast numbers, but in very minute quantities, in the nuclear thread. 

 The supposition of a vast number is not only required by the 

 phenomena of heredity but also results from the comparatively 

 great length of the nuclear thread : furthermore it implies that 

 each of them is present in very small quantity. The vast number 

 together with the minute quantity of the ancestral germ-plasms 

 permit us to conclude that they are, upon the whole, arranged in a 

 linear manner in the thin thread-like loops : in fact the longitudinal 



1 Wilhelm Boux, ' Ueber die Bedeutung der Kerntheilungsfiguren.' Leipzig, 

 1884. 



