THE MECHANISM OF SEX-DETERMINATION 39 



ever, from the female, in that at each of these two di- 

 visions the cells are equal in size. Thus four sperm- 

 cells are produced from each original cell, all four pro- 

 duce tails, and become spermatozoa. 



At the time of fertilization, when the spermatozoon 

 touches the surface of the egg, the egg pushes out a cone 

 of protoplasm at the point of contact (Fig. 19), and, 

 lending a helping hand, as it were, to the sperm, draws 

 it into the egg. The projecting cone of protoplasm 

 is called the fertilization cone. In a few minutes the 

 head of the sperm has entered. Its tail is often left 

 outside. The head absorbs fluid from the egg and 

 becomes the sperm nucleus, which passes towards the 

 center of the egg. Here it comes to lie by the side of the 

 egg nucleus, and the two fuse. The walls of the com- 

 bined nuclei dissolve away and the chromosomes appear. 

 Half of these are derived from the father through 

 the nucleus of the sperm, and half from the mother 

 through the egg nucleus. If we count the paternal 

 chromosomes, there are half as many of them as there 

 are chromosomes in each cell of the body of the father. 

 Presently I shall point out that this statement is not 

 always true, and on this little fact, that it is not quite 

 true, hangs the whole story of sex-determination. 



What is the meaning of these curious changes that 

 have taken place in the egg and sperm ? Why has the 

 egg, before developing, twice thrown away its most 

 valuable heritage — its chromatin material? We do 

 not know with certainty, but one consequence at least 

 stands out clearly ! Before the egg gave off its polar 

 bodies it had the full, or diploid, number of chromo- 

 somes. After this event it has only half as many. A 



