130 F. M. BALFOUR. 



is itself developed from the nucleus of a spermatic cell.i The 

 spermatic cells originate from cells (in the case of Vertebrates at 

 least) identical with the primitive ova, so that the fusion which 

 takes place is the fusion of morphologically similar parts in the 

 two sexes. 



It must not, however, be forgotten, as Strasburger has pointed 

 ^ out, that -that part of the protoplasm of the generative cells of 

 the two sexes also fuse, viz. the tail of the spermatozoon with 

 the protoplasm of the egg. But there is no evidence that the 

 former is of importance for the act of impregnation. The fact 

 that impregnation mainly consists in the union of two nuclei gives 

 an importance to the nucleus which would probably not have 

 been accorded to it on other grounds. 



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 

 impregnation of an egg : — 



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



(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 ger- 

 minal vesicle, these remams 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 ^gg. 



(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 ^g^^. 



' This seems the most probable view with rerereiice to the iiafure of the 

 head of the spermatozoon, though the poiut is not perhaps jet deCnitely 

 decided. 



