46 CENTROSOMES, SPINDLE AND ASTERS [CH. 



existence of exactly similar centrosomes in eggs which 

 develop parthenogenetically. It is usually said that in the 

 fertilised egg the middle piece of the spermatozoon becomes 

 separated from the "head/' and that while the head swells 

 up to form the male pronucleus the middle piece divides, 

 giving rise to the two centrosomes of the mi to tic spindle. 

 This is certainly the simplest interpretation of the figures 

 commonly seen in fertilisation, but detailed observation of 

 the process in certain cases suggests that it may not be a 

 complete account of the process 1 . In the Axolotl, for ex- 

 ample, JENKINSON (1904) found that the middle piece of the 

 spermatozoon, after separating from the "head," swells up 

 and becomes a vacuolated mass or sphere, around which an 

 aster begins to develop, apparently by means of fluid 

 travelling out of the sphere and pushing out elongated 

 alveoli in a radiating manner around it (PI. III). As the 

 sphere enlarges by becoming more and more vacuolated, 

 more distant radiations appear in the surrounding proto- 

 plasm, which JENKINSON attributes to the attraction of fluid 

 from the egg-substance into the swelling sphere. Within the 

 sphere a more solid body now appears, which is the definitive 

 centrosome; this divides, the two halves separate, and a 

 spindle is formed between them, lying between, and to some 

 extent including, the egg- and sperm-nuclei, which are now 

 of equal size and nearly in contact. The nuclear membranes 

 then disappear, the chromosomes come to lie on the spindle, 

 and the mitotic figure of the first segmentation division is 

 definitely established. In the equatorial plate stage the 

 centrosomes then swell and become vacuolated and soon 



1 That the middle piece of the spermatozoon is not completely converted 

 into centrosomes is proved by the discovery of MEVES that in Echinoids a 

 portion of the middle piece remains in the egg-cytoplasm during segmentation 

 and can be traced as far as the 3 2 -cell stage (1912, 1914). 



