=The Fertilized Ovum Termed a Zygote.--=After a number of generations of such asexual reproduction, sexual reproduction occurs. The ova arise as usual. Certain members of the colony, on the other hand, go to the other extreme and divide up into bundles of from sixty-four to one hundred twenty-eight minute slender cells, each provided with flagella for locomotion. When mature these small flagellate cells, now known as _spermatozoa_, escape into the interior of the parent colony and swim about actively. Ultimately each ovum is penetrated by a spermatozoon, the two cells fuse completely and thus form the single _fertilized ovum_ or _zygote_. The body-cells of the mother colony finally disintegrate. After a period of rest each zygote, through a series of cell-divisions, develops into an adult Volvox. In some species of Volvox a still further advance is seen, in that instead of both kinds of gametes being produced in the same colony, the ova may be produced by one colony and the spermatozoa by another. Here, then, we have the foreshadowings of two sexes as separate individuals, a phenomenon of universal occurrence among the highest forms of animal life. [Illustration: FIG. 5 _Volvox globator_ (from Hegner after Oltmanns). Half of a sexually reproducing colony: _o_, eggs; _s_, spermatozoa.] =Advancement Seen in the Volvox Colony.--=In the Volvox colony there is a distinct advance over the conditions met with in various lower protozoan colonies in that only certain individuals of the colony take part in the process of reproduction and these individuals are of two distinct types; one is a larger, food-laden cell or egg and the other a small, active, fertilizing cell. The motile forms are produced in much greater numbers than the eggs, plainly because they have to seek the egg and many will doubtless perish before this can be accomplished. This disparity in number is only a means of insuring fertilization of the egg. The remaining cells of the body carry on the ordinary activities of the colony such as locomotion and nutrition and have ceased to take any part in the production of new colonies.