50 CENTROSOMES, SPINDLE AND ASTERS [CH. 



with the names of BUTSCHLI, FLEMMING, VAN BENEDEN, 

 HEIDENHAIN, not to mention many more recent workers. 

 The second, or archoplasm hypothesis of BOVERI, and its 

 more recent modifications, are supported by the fact that 

 in many cases, especially when the surrounding protoplasm 

 has an alveolar structure or contains yolk, the rays from 

 the asters can be seen growing from the centres in the walls 

 of the alveoli or between the yolk-granules, and are obvi- 

 ously distinct from the protoplasm in which they lie. They 

 appear more dense, stain more deeply, and give the impres- 

 sion of consisting of a different substance. Moreover, at the 

 close of mitosis, when the cell divides, the remains of the 

 spindle and asters may contract into a mass which often 

 has a somewhat fibrous structure and which persists for 

 some time. This mass is often known by its German name 

 of Spindelrestkorper, and by some writers as spindle-bridge, 

 and is sometimes spoken of as the Nebenkern, but this latter 

 is an unfortunate word, since it has been applied to bodies 

 of quite distinct nature and origin, and has often been used 

 for the "mitochondrial" body which will be described at a 

 later stage. The fact, however, that in some cases at least 

 not only are the astral rays and spindle-fibres apparently 

 quite distinct from the surrounding protoplasm, but also 

 that their remains persist for a considerable period after 

 cell-division is completed, is rather strongly in favour of 

 the view that they are not mere rearrangements of the 

 ordinary cytoplasmic structure, but are composed of a 

 somewhat distinct substance. It is possible, of course, that 

 the truth is represented by a compromise between these 

 two hypotheses that the asters and spindle-fibres are 

 formed by a modification of the general cytoplasm, and 

 that the extent of this modification varies in different cases, 

 so that in some cells the fibres consist of protoplasm hardly 



