30 CELL DIVISION [CH. 



single, and may appear as a mere stained granule just 

 outside the nucleus, or may be surrounded by a zone of 

 protoplasm differing somewhat from the remaining cyto- 

 plasm of the cell. As the chromosomes are being formed 

 inside the nucleus, the centrosome divides; the halves 

 separate and travel to opposite poles of the nucleus. Here 

 again two somewhat different types of behaviour may be 

 found in different cases. Sometimes, as the daughter-centro- 

 somes separate, a sheaf of protoplasmic threads is seen 

 between them, forming a spindle-shaped structure and con- 

 verging on the centrosomes at either end, the whole being 

 outside the nucleus. In this case, as the centrosomes di- 

 verge, that part of the nuclear membrane nearest to the 

 sheaf or spindle of threads disappears, with the result that 

 when the centrosomes have reached points at opposite sides 

 or poles of the nucleus, the threads pass completely across 

 the nucleus, which is now represented only by a group of 

 chromosomes, the membrane having entirely disappeared. 



In the second type the centrosomes pass to opposite poles 

 of the nucleus before any dissolution of the nuclear mem- 

 brane takes place, and in this case no continuous spindle 

 of threads is formed until rather later (PL II, A, B, C). 

 When the centrosomes reach the opposite poles, the nuclear 

 membrane gradually dissolves, and as it does so a sheaf of 

 fibres is formed across it from centrosome to centrosome. 

 In this second case, therefore, the spindle-fibres seem to be 

 formed at least partially from within the nucleus, while in 

 the former they were entirely extra-nuclear in origin. 



At the same time that the spindle-fibres connecting the 

 two centrosomes appear, radiating strands or fibres are also 

 seen running out as rays from the centrosomes into the sur- 

 rounding cytoplasm. These rays are often quite similar to 

 the spindle fibres except that they taper off and disappear 



