32 INTRODUCTORY I 



The definitive centrosome now divides at right angles to 

 this second part of the path, and in a plane parallel to the 

 equator of the egg. Preceded by its two centrosomes and 

 the sperm-aster, the sperm-nucleus continues to move towards 

 the female pronucleus. When they meet, the line joining 

 them, that is the second part of the sperm-path, is naturally at 

 right angles to the line uniting the two centrosomes (Fig. 8). 



The ovum itself has no centrosome, and it is an invariable 

 rule for the female centrosome. even though present in the 

 polar divisions, to disappear. There is also little doubt that 

 in all cases the definitive centrosome is of male origin, though 

 whether derived from the centrosome present in the middle 

 piece, or developed de novo, as in the Axolotl, is not certain. 

 It may be said, however, that while there is very little 

 evidence for the continued persistence of the original centro- 

 some in any case, a conclusive demonstration has been given 

 recently by Lillie of the origin of the cleavage centrosome 

 from the sperm-nucleus, in Nereis. For in Nereis, first the 

 middle piece is normally left outside with the tail, but a 

 centrosome nevertheless is seen later in the sperm-aster ; and 

 secondly Lillie has shown by a very beautiful experiment that 

 any fragment of sperm-head introduced into the egg rotates, 

 and develops at its inner end an aster and a centrosome. The 

 experiment consists in centrifuging the egg during the entrance 

 of the sperm. The jelly, being lighter than the egg itself, is 

 dragged off towards the centripetal end of the tube, and 

 removes with itself any part of the spermatozoon that may 

 still be outside the egg. It is possible in this way to remove 

 the hind end of the head but allow the front end to go in. 

 The part that enters behaves as described. Sometimes it is 

 by the violence of the operations divided into two : each piece 

 then rotates, and forms aster and centrosome. 



With the evidence before us it would hardly be too rash to 

 suppose that it is a general rule for the definitive centrosome 

 to be not only a sperm-centrosoine, but one deyeloped de novo 

 from the sperm-nucleus. 



4. The pronuclei now lie side by side between the two 

 centrosomes. From the latter spindle-fibres grow out and 



