86 SUMMARY. 



head of the spermatozoon which, as stated above, contains part 

 of the nucleus of the primitive spermatic cell. The spermatic 

 cells originate from cells indistinguishable from the primitive 

 ova, so that the fusion which takes place is the fusion of morpho- 

 logically similar parts in the two sexes. 



These conclusions tally very satisfactorily with the view 

 adopted in the Introduction, that impregnation amongst the 

 Metazoa was derived from the process of conjugation amongst 

 the Protozoa. 



Summary. 



In what may probably be regarded as a normal case the 

 following series of events accompanies the maturation and im- 

 pregnation of an ovum : 



(1) Transportation of the germinal vesicle to the surface of 

 the egg. 



(2) Absorption of the membrane of the germinal vesicle 

 and metamorphosis of the germinal spot and nuclear reticulum. 



(3) Assumption of a spindle character by the remains of 

 the germinal vesicle, these remains being probably in part 

 formed from the germinal spot. 



(4) Entrance of one end of the spindle into a protoplasmic 

 prominence at the surface of the egg. 



(5) Division of the spindle into two halves, one remaining 

 in the egg, the other in the prominence ; the prominence becom- 

 ing at the same time nearly constricted off from the egg as a 

 polar cell. 



(6) Formation of a second polar cell in the same manner as 

 the first, part of the spindle still remaining in the egg. 



(7) Conversion of the part of the spindle remaining in the 

 egg into a nucleus the female pronucleus. 



(8) Transportation of the female pronucleus towards the 

 centre of the egg. 



(9) Entrance of one spermatozoon into the egg. 



(10) Conversion of the head of the spermatozoon into a 

 nucleus the male pronucleus. 



(i i) Appearance of radial striae round the male pronucleus, 

 which gradually travels towards the female pronucleus. 



