546 MATURATION AND IMPREGNATION OF THE OVUM. 



Hertwig's discovery is in no way opposed to Mr Darwin's 

 theory of pangenesis and other similar theories, but does not 

 afford any definite proof of their accuracy, nor does it in the 

 meantime supply any explanation of the origin of two sexes or 

 of the reasons for an embryo becoming male or female. 



Summary. 



In what may probably be regarded as a normal case the 

 following series of events accompanies the maturation and im- 

 pregnation of an egg : 



(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. 



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

 germinal vesicle, these remains being probably largely 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 

 becomes at the same time nearly constricted off from the egg as 

 a polar cell. 



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

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



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

 egg after the formation of the second polar cell 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. 



(u) Appearance of radial striae round the male pronucleus 

 which gradually travels towards the female pronucleus. 



(12) Fusion of male and female pronuclei to form the first 

 segmentation nucleus. 



