on. LVTII.] IMPREGNATION. 805 



loses its investing membrane, and undergoes the changes associated 

 with karyokinesis. In some cases, however, the chromosomes do 

 not undergo longitudinal splitting, but divide into two equal 

 groups which form the bases of the two new nuclei. The spindle 

 lies at first horizontally under the outer circumference of the cell. 

 At each end of the spindle an attraction sphere is situated; the 

 granules of the protoplasm take a radial arrangement around the 

 two attraction spheres. The spindle then becomes vertical, and 

 the nucleus divides into two ; the upper daughter nucleus with a 

 thin investment of protoplasm is extruded from the body of the 

 cell. The other daughter nucleus remains within the ovum ; it 

 does not return to the resting stage, but proceeds at once to 

 repeat the process, and a second polar globule is extruded, so 

 that only a quarter of the original nucleus remains within the 

 ovum ; this is then called the female pronucleus, and it travels 

 towards the centre of the ovum. The first polar globule often 

 divides into two after it is extruded. 



Simultaneously the ovum shrinks, not only from the loss of the 

 polar globules ; but also from a shedding out of liquid which 

 collects between the ovum and the zona pellucida and is called 

 the perivitelline fluid. In this, later on, spermatozoa which have 

 penetrated the zona pellucida, may be seen swimming. 



Impregnation. 



We now have a somewhat shrunken ovum possessing a female 

 pronucleus. Impregnation or fertilization consists in the embed- 

 ding of the head (nucleus) and middle portion of one spermatozoon 

 in the protoplasm of the ovum ; the tail is lost, and the male 

 attraction sphere originally contained in the middle piece of the 

 spermatozoon has around it the same star-like arrangement of the 

 protoplasmic granules of the ovum which we described around 

 the female attraction sphere. The head of the spermatozoon 

 is now called the male pronucleus ; it travels to the female pro- 

 nucleus, and in some animals may for a time leave a distinct 

 channel marking its pathway. Having reached the female pro- 

 nucleus, it fuses with it and forms an ordinary nucleus. The 

 whole cell so produced is then often called the blastosphere. 



A great deal of discussion has taken place as to the meaning 

 of the discharge of the polar globules. Fertilization consists in 

 bringing to the ovum of a certain amount of germinal plasma 

 from another individual or male, and Weismann assumes that it 

 is necessary that the ovum, prior to development, should get rid 

 both of its old histogenetic plasma, and of so much germinal 

 plasma (i.e. matter endowed with heredity) as may be brought to 



