THE CELL AND PROTOPLASM. 127 



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 

 reproductive 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 centrosome enters the egg, forming what 

 is called the male pronucleus (Fig. 35-38, mri). 

 This entrance of the male cell occurs either 

 before the formation of the polar cells of the 

 egg or afterward. If, however, it takes place 

 before, the male pronucleus simply remains dor- 

 mant 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 

 describing, it is certain that the egg centrosome 

 disappears while that of the spermatozoon is re- 

 tained alone to direct the further activities (Fig. 

 41). Possibly this may be the case in all eggs, 

 but it is not sure. It is a matter of some little 



