[Illustration: FIG. 63. _Volvox aureus_, after Klein and Schenck. _A_, besides the small flagellate somatic cells of the colony there are five large egg-cells (_t_) which are capable of parthenogenetic development, three recently fertilized egg-cells (_o_) and a number of male germ-cells (_a_) in process of multiplication. From each of these, by continued division, a bundle of spermatozoa arises. _B_, a bundle of thirty-two sperm-cells in process of development, seen from above. _C_, the same seen from the side. Magnified 687 times. _D_ individual spermatozoa, magnified 824 times.] In _Volvox_ the state of matters is similar to that in _Eudorina_; here again, in addition to the 'asexual' reproduction through the 'Parthenogonidia' which are like egg-cells in appearance (Fig. 63, _A_, _t_), there are also male and female germ-cells which are usually produced alternately with the former, but sometimes at the same time, as in Fig. 63. The egg-cells are large and have no flagella, the sperm-cells lie together in clusters, and after they are mature (_D_) they swim freely in the water and then bore into another colony, where each unites with an egg-cell. The difference between the two kinds of cells consists therefore in the much greater number, the much smaller size, and the greater activity of the male cells, and in the smaller number but much larger size of the female cells--a differentiation in accordance with the principle of division of labour, depending on the fact that the two kinds of cells must reach each other, and yet must contain a certain mass of living protoplasm. While the small size but large number of male cells, combined with their motility, gives them an advantage in seeking out and boring into the female cells, the large size of the latter, on the other hand, makes up for the loss in mass which the fertilized egg would otherwise suffer from the diminution in size of the male cell. This difference in size may be greatly accentuated; thus in one of the brown sea-wracks, for instance, the spermatozoa are only 5 micro-millimetres in length, while the ova are spherical and have a diameter of 80-100 micro-millimetres, thus containing a mass 30-60,000 times greater (Möbius). Fig. 64 shows an ovum of this species surrounded by spermatozoa.