^94 



Mr. L. Doncaster, 



[Sept. 21, 



and very frequently the chromatin is closely packed round the nuclear 

 membrane, especially at one pole, with fine strands radiating from this 

 towards the more empty pole. At this stage they are much like the 

 spermatids, in which the chromatin has a similar arrangement (cf. figs. 8 c 

 and 17), but the nucleus is much larger, having the same size as that of the 

 primary spermatocytes. 



The chromatin next becomes grouped in the form of large elongate granules 

 or small bands scattered under the nuclear membrane ; their number is not 

 easy to determine, but it approaches 20 (fig. 9). They then become combined, 

 but whether by end-to-end pairing I cannot determine, into 10 very definite 

 long bands having a more or less regular meridional arrangement under the 

 membrane (fig. 10 a, h, c). 



In one testis, in which a single follicle is much retarded, all the others 

 containing well advanced spermatids, there are about 20 of these bands in 

 each nucleus, and as they concentrate themselves to form chromosomes they 

 ^ippear to pair side by side to form 10 split chromosomes (Plate 1, figs. 18 a, b). 

 It is possible that this lagging follicle is abnormal, for in all other cases where 

 the bands can be counted their number is clearly about 10. The bands then 

 become shorter, and arrange themselves across the nucleus in a fairly regular 

 equatorial i)late, in which 10 chromosomes radiate from a centre (figs. 11, 12). 



The nucleus has now assumed the form of a wide spindle, stretching almost 

 from end to end of the cell, but the membrane, though faint, appears still to 

 persist. At each end of the nucleus a minute centrosome may be seen, and 

 apparently within the nucleus is a system of spindle fibres extending from 

 the centrosomes to the chromosomes, which lie across the centre. Commonly 

 also at this stage a small deeply-stained dot may be seen outside the nuclear 

 spindle, generally nearer one end. It is shown in figs. 18, 14, 15 ; whore, as 

 in figs. 13 6, 14 «, it appears witliiu the spindle, it is in reality at a difi'ereiit 

 level, and is always quite near the edge of the cell. Tlie metaphase condition 

 ap])ears to last for some little time, for it is frequently found (fig. 13 a, h) 



In the early anaphase the nuclear membrane has (piite disappeared, and the 

 chroiriosomes ap])ear to split longitudinally, for in the equatorial ])late they 

 are arranged with their length across the s))indle. I'lie cell elongates some- 

 what, and the spindle extends coni))letely from end to end. In early 

 anaphase the chromosomes have the form of short rods converging towards 

 the centrosomes (fig. 14 a, i); as they move ajjart lli(!y shoi ten, and as 

 they aggregate round the centrosome they become still more concentrated. 

 The cell meanwhile becomes constricted, a sheaf of spindle fibres extending 

 from one daughter nucleus to the other, connecting the two halves (fig. 15 a, b). 

 Tlie two grou]»Hof chromosomes round themselves ofT into nuclei, having al)out 



