THE FUNCTION OF THE CENTROSOME. 



211 



ration divisions do not involve the sperm-centrosomes, but are 

 separate and independent of them. The first maturation 

 amphiaster in certain eggs, e.g., Thalassema, Nereis, and Myzo- 

 stoma, is formed only after the sperm enters, and yet its cen- 

 trosomes are not brought in by the sperm nor do they arise 

 near it. In Chaetopterus, on the other hand, a complete 

 amphiaster with centrosomes, centrospheres, astral rays, and 

 spindle fibers is developed and remains for hours in the meta- 



FiG. 4. — Diagram indicating the different stages of maturation attained by the eggs of 

 various animals before the sperm enters. A, Nereis, Tlialassema ; B, Chaetopterus 

 and some other marine annelids ; C, frog, Axolotl, lamprey ; D, sea-urchin. 



phase, if the egg is left unfertilized in sea-water; and the same 

 appears to be true of many other marine annelids. This elab- 

 orate machinery of mitotic division is immediately set in 

 motion upon the entrance of the spermatozoon, though the 

 sperm and its centrosomes are in a distant portion of the egg. 

 All the phases of this and the subsequent mitosis are inde- 

 pendent of the karyokinetic changes in the vicinity of the 

 sperm. 



Since in one form the oocyte will not divide until the sperm 

 enters the cell, even though the centrosomes and the whole 



