56 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



in and attach themselves to the chromosomes or meet and form the 

 central spindle fibers extending from pole to pole. 



The figures of Swingle (920) seem decidedly to favor the view 

 that in Stypocaulon the spindle fibers form new connections with the 

 chromosomes by growing in from the poles at each mitosis. The same 

 is true of the spindle formation in the first division of the tetraspore 

 mother cell of Dictyota, as described by Mottier (67) . On the other hand, 

 the same author's figures of spindle formation in the second division 

 seem to agree closely with the corresponding stages in the ascus. 



PERMANENCE OF THE CHROMOSOMES. 



It is evident that the resting reticulum of the nucleus of the mildew 

 contains at least two elements, and that the chromosomes of the equa- 

 torial-plate stage are formed from the chromatin of the resting and 

 spirem stages by the segregation of a less stainable material — the linin — 

 and a denser, more stainable material. The latter becomes condensed 

 into the oblong chromosomes connected to the central body by bundles 

 of achromatic threads. The formation of the spirem is a process in 

 which the principal strands of the reticulum come more prominently 

 into view as a result of their contraction while attached at one end to the 

 central body. In this process they lose the fibrillse by which they were 

 connected to form the resting reticulum and become sharply defined, 

 highly stainable threads lying in an entirely achromatic nuclear sap. 



The material of each chromosome is still distributed at the spirem 

 stage through the whole length of the strand out of which it is later 

 segregated. The withdrawal of this chromatic material into the com- 

 pact body of the chromosome leaves the spirem strand as a thin achro- 

 matic fiber or bundle of fibers connecting the chromosomes to the pole. 

 Whether in the division of the central body and separation of the 

 daughter centers to form the poles of the spindle the achromatic fibers 

 are split longitudinally or are merely separated into two bundles is not 

 clear from my preparations. It is, however, plain that each chromo- 

 some is from the start connected with both poles, and that provision is 

 thus made for the separation of the daughter chromosomes and their 

 withdrawal to the poles of the spindle to form the daughter nuclei. 



During the separation of the daughter centers no so-called central 

 spindle is present, but in the anaphases central spindle fibers running 

 through from pole to pole are conspicuous ; and they persist until the 

 daughter nuclei have begun their independent development, being then 

 apparently gradually disintegrated. How these central spindle fibers 

 arise as distinct from the fibers which draw the chromosomes to the 

 poles is not clear. 



