The Spermatogenesis in Pentatoma up to the Formation of the Spermatid. 55 



But the chromatin nucleolus, on the contrary, is sometimes not so 

 constricted. 



The arrangement of the chromosomes in the equatorial plate is 

 best seen on pole views. Of these are given a number of figures 

 (Figs. 176 — 189), which have been drawn in succession as I chanced 

 to meet with them on sections , so that from them the general 

 arrangement of the chromosomes may be deduced. As they show, 

 it is most usual to find a circle of 6 chromosomes, within which lies 

 the 7th chromosome and the chromatin nucleolus , though deviations 

 from this arrangement are frequent. These figures show the 

 greatest diameter of each chromosome seen in the plane of the 

 section; but the differences in the volume of individual chromosomes 

 can only be determined with accuracy on lateral views of the spindle. 

 The chromosomes are rarely in actual contact with one another, so 

 that it is probably the fibres of the central spindle which keep them 

 apart from one another. The size of the spindle varies more' or less 

 according to the size of the cell. 



c) The 1st spermatocytic metakinesis and dyaster. 



Though the reduction phenomena are exactly the same in cells 

 of the small and of the large generation , they may be studied to 

 better advantage in the latter, since in these the centrosomes are larger, 

 and, owing to the greater size of the cell body, the spindle fibres 

 and their movements may be seen with greater clearness; the figures 

 given to represent these phenomena have been drawn mainly from 

 cells of the larger generation. 



The 7 chromosomes and the chromatin nucleolus are so situated 

 in the monaster stage, that their transverse constrictions coincide 

 with the plane of the equator (Figs. 169, 172 — 174), These elements 

 first undergo a slight degree of elongation, probably due to the tension 

 of the mantle fibres ; and shortly after follows a transverse constriction 

 of them (Figs. 190 — 195). The division of the chromosomes must be 

 considered to be due to automatic movements of the chromatin microsomes, 

 as has already been discussed; while the separation of their halves is, of 

 course, produced by the tension of the mantle fibres. If the divison 

 of the chromosomes were directly caused by the mantle fibres, one 

 would expect to find the chromosomes becoming gradually more and 

 more constricted, until the constriction were reduced to a mere thread; 

 this,, however, is rarely observed (Fig. 191). On the contrary, two 

 different modes of division of the chromosomes can be observed. Either 

 a narrow split, with parallel boundaries, extends across the chromo- 



