[Illustration:  FIG. 38—­First division complete and first polar cell formed, pc’.]

[Illustration:  FIG. 39.—­Formation of the second polar cell, pc".]

[Illustration:  FIG. 40.—­Completion of the process of extrusion of the chromatic material; fn shows the two chromosomes retained in the egg forming the female pronucleus.  The centrosome has disappeared.]

Previously to this process the other sexual cell, the spermatozoon, or male reproductive cell, has been undergoing a somewhat similar process.  This is also a true cell (Fig. 34, mc), although it is of a decidedly smaller size than the egg and of a very different shape.  It contains cell substance, a nucleus with chromosomes, and a centrosome, the number of chromosomes, as shown later, being however only half that normal for 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.

[Illustration:  FIG. 36—­The egg centrosomes have changed their position.  The male cell with its centrosome remains inactive until the stage represented in Fig. 42.]

[Illustration:  FIG. 37—­Beginning of the first division for removing superfluous 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 (Figs. 35-38, mn).  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 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 describing, it is certain that the egg centrosome disappears while that of the spermatozoon is retained 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 interest to have this settled, for if it should prove true, then it would evidently follow that the machinery for cell division, in the case of sexual reproduction, is derived from the father, although the bulk of the cell comes from the mother, while the chromosomes come from both parents.