154 Edward Hindle 



About half an hour after transference to normal sea water, from 

 one pole of the nucleus a definite aster begins to develop, its rays 

 focussing in a more or less indistinct centrosome situated on the 

 nuclear membrane (Fig. 4). By division of the centrosome a typical 

 amphiaster is formed in the nuclear area and as it develops the 

 nuclear membrane disappears (Fig. 15). At the same time the 

 chromatin assumes the form of a spireme, which subsequently breaks 

 up into about 18 long and slender chromosomes. At this stage it is 

 impossible to clearly distinguish their number, but, as the chromo- 

 somes are gradually drawn into the equator of the cleavage amphi- 

 aster (Fig. 6), they shorten considerably and become quite distinct 

 by the time that the equatorial plate is formed (Fig. 5). 



At this stage we have made numerous counts of the chromo- 

 somes and invariably found it in the neighbourhood of 18, which is 

 half the number that is normally present in this species. 



During the metaphase each chromosome splits longitudinally, and 

 in the succeeding anaphase the halves move away from each other 

 along the spindle fibres, 18 going to each pole (Fig. 7). As they 

 approach the centrosomes the chromosomes swell up and become 

 indistinct, and finally fuse together to form two daughter nuclei. 

 These changes are accompanied by a division of the cytoplasm into 

 two equal parts, followed by complete disappearance of the spindle 

 fibres, and the first cleavage is complete (Fig. 8). 



Each daughter nucleus remains partially surrounded by a clear 

 region that seems to represent the centrosome together with a 

 slight accumulation of cytoplasm, and here the cleavage aster of 

 the next division first makes its appearance. After the first cleav- 

 age is complete the succeeding divisions follow each other in re- 

 gular succession and present all the features of ordinary mitosis 

 (Fig. 9). 



In each case the nucleus becomes completely surrounded by a 

 clear zone of hyaloplasm. This is followed by a period of nuclear 

 growth during which this perinuclear zone almost disappears. From 

 the centrosome, that has persisted at one pole of the nucleus, rays 

 now appear extending out into the cytoplasm (Fig. 10) and by the 

 subsequent division of the centre of attraction a typical amphiaster 

 is produced (Figs. 9 10). As it develops the nuclear membrane 

 disappears and the chromatin breaks up into 18 chromosomes which 

 arrange themselves about the equator of the amphiaster (Fig. 9). 

 In this way a typical mitotic spindle is formed in the nuclear area 



