42 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



conspicuous attachment of the chromatin threads to the central body we 

 have a satisfactory explanation of the unipolar structure of the spirem 

 nucleus. Flemming's schematic figures of the spirem of the sala- 

 mander are, in the relation shown between the center and the chromatin 

 loops, strikingly similar to the figures shown in the ascus ; but no such 

 conspicuous connection between the center and the chromatin strands 

 as is seen in the ascus could be demonstrated by Flemming in the cells 

 of the salamander. 



The number of strands of chromatin at this stage can be determined 

 with great certainty. In practically every figure which was studied 

 careful focusing shows that there are just eight threads passing back 

 from the center into the antipolar region. Near the center, of course, 

 they may in some cases overlie and obscure each other, but by tracing 

 them back a short distance toward the antipolar region the number can 

 be made out with unfailing regularity. The number of these strands 

 coincides, as we shall see, with the number of chromosomes in the equa- 

 torial plate, and the conclusion seems entirely certain that each of the 

 strands corresponds to a single chromosome, and that thus each chro- 

 mosome has a permanent attachment to the central body. 



A further, rather long, period intervenes between the stage of the 

 fully developed spirem and the equatorial plate of the first division. 

 The perithecium and the asci continue to grow slowly in size, but more 

 marked than the growth in size at this period is the differentiation 

 which occurs in the perithecial cells. The penicillate cells proceed to 

 their fullest size and differentiation as described above. The append- 

 ages are developed and the differentiation of the outer, middle, and 

 inner zones of the perithecial envelopes becomes more apparent. 



The nucleus remains for some time at the base of the ascus, where 

 the latter narrows to form the short stalk; but as it passes on in its 

 development toward the formation of the chromosomes and the spindle, 

 it generally migrates to a region higher up and nearer the middle of the 

 enlarged portion of the ascus. The further differentiation of the chro- 

 mosomes now continues. The process seems to be as follows : A more 

 densely staining portion of each thread becomes differentiated at some 

 point in its length, forming an elongated and bent rod-shaped body, 

 which is to become the chromosome. The process of differentiation 

 seems to consist in the drawing together and the aggregation at some 

 point of the more densely staining constitutents of the strand. At the 

 same time a contraction or shortening of the whole thread occurs. The 

 segregation of the more stainable portions of the thread into the chro- 

 mosomes leaves an achromatic filament or bundle of fibrillse connecting 



