CELLS IN DIVISION 



disappeared after mitosis is completed. This was mainly derived from 

 studies in the eighteen-eighties on fertilization and cleavage in 

 Ascaris. In the description of the cleavage cycle in Ascaris given by 

 VAN Beneden and NeyTj^o* they showed that in anaphase the 'cor- 

 puscule polaire' has already divided within the 'sphere attractive', 

 which itself follows suit in early interphase. Both division centres are 

 then ready for the next mitosis (Figure 37).* The modern usage of 

 centriole and centrosome was established by Boveri^o^ in 1901, though 

 as Wilson^"^ points out, the latter term has since been used in several 

 senses. 



The observation that in some instances the centriole was a permanent 

 cell organ persisting from one cell generation to the next led to the 



Figure 37 Stages in the first cleavage of the egg oi Ascaris megalocephala to 

 show division of the central body, a ^ and ^ pronuclei not yet fused; 

 only one pronucleus is shown, c The centrioles divide during anaphase. 

 d & e The centrospheres divide after cleavage is completed, f Interphase 

 following second cleavage. From van Beneden and Neyt^**. 



hypothesis that an aster could only develop round a centriole derived 

 by division from the original centriole of the fertilized t^g. On 

 BovERi's theory of fertilization, this was contributed by the sperm. 

 Not all cytologists have been able to agree that the granule at the astral 

 centre is a permanent cell organ, which alone can evoke a radial 

 arrangement of the surrounding cytoplasm. Within both unfertilized 

 and fertilized eggs, adventitious monasters with centrioles can be 



* Lewis*"** has followed the division of the centrosome and the migration of the daughter 

 centres over the nuclear membrane in a living egg of a monkey before the prophase of the 

 first cleavage. 



107 



