iv] THE CENTROSOME 45 



is probably most convenient to use the former terminology 

 to designate the whole structure as a centrosome and to 

 use the word centriole for the central granule when it is 

 present. Using the words, then, in this sense, it is found 

 that the centrosome shows considerable variation in form, 

 not only in different species of animals, but also in the 

 different stages of one cell-division. Not rarely a centrosome, 

 which at first has the form of a small rather dense mass with 

 central granule, swells up, as the nuclear division proceeds, 

 -and becomes greatly enlarged; the granule may become 

 vacuolated and disappear, and the whole body take the 

 form of a considerable mass of vacuolated protoplasm from 

 the edges of which the astral rays and at one point the 

 spindle-fibres radiate (cf. PI. II). Then a fresh concentration 

 may take place in the centre, and a new granule or centriole 

 may appear within it, and if the cell is undergoing repeated 

 rapid division this new centriole may divide in the anaphase 

 or telophase of one nuclear division in preparation for the 

 next division which is to follow it. Sometimes the whole 

 organ has a concentric structure, a central granule enclosed 

 in dense protoplasm, and outside this a clear space, which 

 is in turn surrounded by a more dense zone from which the 

 astral rays arise; or some of these rays may originate from 

 the central mass and cross the intervening clear space, 

 while the remainder radiate from a zone nearer the cir- 

 cumference. 



From the preceding account it will be seen that both the 

 origin of the centrosome and its relation to the asters and 

 spindle of the mitotic figure are still somewhat uncertain. 

 There can be no doubt that in the first segmentation 

 division of many eggs the centrosomes arise in connection 

 with the middle piece of the spermatozoon, but that this is 

 not the only possible method of origin is proved by the 



