Sexual Reproduction and the Organization of 

 THE Nucleus in Certain Mildews. 



By r. a. harper. 



INTRODUCTION. 



It is one of the best-established facts regarding mitosis that from 

 certain late prophase stages until the formation of the diaster a specially 

 evident connection between the chromosomes and the spindle poles is 

 maintained by means of conspicuous fibrillar structures. The existence 

 of these apparent fibrillae is generally admitted even by those who accept 

 a so-called dynamic theory of the central body as opposed to the concep- 

 tion of contractile spindle and polar fibers. It is evident, also, that this 

 connection influences, if not directly determines, the motions of the 

 chromosomes and their separation into two groups to form the daughter 

 nuclei. This is equally true for the cells of animals and plants and for 

 cases of multipolar as well as bipolar spindles. 



Rabl was the first to point out that in the spirem, and even in the 

 earlier resting-stages, some influence is exerted on the nuclear contents 

 which gives a definite polar arrangement to the chromatic elements. 

 This centering of the spirem loops on a polar field has come to be com- 

 monly known as Rabl's (78) figure. The same arrangement of the 

 spirem was found by Strasburger (88) in both the dispirem and the 

 prophases of dividing plant-cells, and its regular occurrence has since 

 been described and figured by numerous authors and for widely sepa- 

 rated organisms. 



The discovery of the so-called attraction sphere and its relation to 

 the karyokinetic figure by Van Beneden (6,7) gave a further impulse to 

 the doctrine of polarity, both in the cell and nucleus, and in a further 

 brief paper (79) Rabl accepted Van Beneden's conception of an attrac- 

 tion sphere and gave a fuller statement of the doctrine that, not only 

 during the process of division as shown by Van Beneden, but in the 

 so-called resting stages, the whole structure of the cell and nucleus is 



NoTB. Numbers inclosed in parentheses refer to the numbers in the litera- 

 ture list at the end of this paper. 



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