in] CHROMOSOMES 29 



formed, the chromatin becomes aggregated upon the thread 

 until it appears either continuous or often divided into 

 small masses like a string of beads ("chromomeres"), and 

 not rarely these granules may be seen to be double, indicat- 

 ing the beginning of a longitudinal split along the thread; 

 the thread then contracts and thickens and becomes seg- 

 mented transversely into a series of short pieces, which are 

 often loop-shaped. These portions are then chromosomes, 

 and their subsequent behaviour is the same as when they 

 have arisen separately from their first appearance. The 

 two apparently distinct modes by which the chromosomes 

 emerge from the nuclear network differ only superficially, 

 depending, no doubt, simply on whether the clear separation 

 takes place early or later, and even when the spireme 

 appears to have the form of a continuous thread it is quite 

 probable that the chromosomes are really distinct. In their 

 final form chromosomes produced by segmentation of a 

 spireme are often, but not always, elongated and loop- 

 shaped, while those formed by concentration around 

 separate centres are usually shorter. 



During the later stages of the formation of the chromo- 

 somes other important changes have been in progress, 

 leading to the formation of the "mitotic spindle" or 

 mechanism by which the chromosomes after division are 

 separated into the daughter nuclei. During the earlier 

 stages of preparation for division a small granule, which 

 stains deeply with chromatin stains, appears outside the 

 nuclear membrane. This granule is called the centrosome or 

 centriole 1 , about the origin and nature of which there has 

 been considerable controversy. The centrosome is at first 



1 There is some confusion in the use of these words, which will be further 

 discussed in the next chapter, where the nature of the centrosome and mitotic 

 spindle will be considered. 



