108 THE STORY OF THE LIVING MACHINE. 



the ordinary cells of the animals. The study 

 of the development of the spermatozoon shows 

 that it has come from cells which contained the 

 normal number of four, but that this number has 

 been reduced to one half by a process which is 

 equivalent to that which we have just noticed in 

 the egg. Thus it comes about that each of the 

 sexual elements, the egg and the spermatozoon, 

 now contains one half the normal number of 

 chromosomes. 



Now by some mechanical means these two re- 

 productive cells are brought in contact with each 

 other, shown in Fig. 34, and as soon as they are 

 brought into each other's vicinity the male cell 

 buries its head in the body of the egg. The tail 

 by which it has been moving is cast off, and the 

 head containing the chromosomes and the centro- 

 some enters the egg, forming what is called the 

 male pronucleus (Fig. 35-38, mri). This entrance 

 of the male cell occurs either before the forma- 

 tion of the polar cells of the egg or afterward. 

 If, however, it takes place before, the male pronu- 

 cleus simply remains dormant in the egg while 

 the polar cells are being protruded, and not until 

 after that process is concluded does it begin 

 again to show signs of activity which result in the 

 cell union. 



The further steps in this process appear to be 

 controlled by the centrosome, although it is not 

 quite certain whence this centrosome is derived. 

 Originally, as we have seen, the egg contained a 

 centrosome, and the male cell has also brought 

 a second into the egg (Fig. 35, ce). In some 

 cases, and this is true for the worm we are de- 

 scribing, it is certain that the egg centrosome 

 disappears while that of the spermatozoon is re- 



