370 ON THE NUMBER OF POLAR BODIES AND [VI. 



only half the number contained in the original nucleus. After 

 Roux's ' elaborate review of the whole subject, we need no 

 longer doubt that the complex method of nuclear division, 

 hitherto known as karyokinesis, must be considered not merely 

 as a means for the division of the total quantity of nuclear sub- 

 stance, 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 perhaps 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 

 discover}'^ 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 pro- 

 duced b}' the longitudinal splitting 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 dis- 

 tribution of the different qualities of the nuclear thread, without 

 exception, in both daughter-nuclei. But these different 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 com- 

 paratively 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 an- 

 cestral 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 splitting of these loops appears to 



* Wilhelm Roux, ' Ueber die Bedcutung der Kerntheilungsfiguren.' 

 Leipzig, 1884. 



