REPRODUCTION BY GERM-CELLS 271 



difierentiation, since it may be absent, but that this distinction of 

 female and male cells is only of secondaiy moment. From the fact 

 that the egg-cells are larger and less active, the 'sperm-cells' or 

 zoosperms smaller and livelier, we can already anticipate what will be 

 more definitely established as our knowledge of the facts increases— 

 that a diflferentiation according to the principle of division of labour 

 has taken place even in the germ-cells, and that the first e9"ect of this 

 is to render the meeting of the cells destined for conjugation easier 

 and more certain. The much smaller and more slender zoosperms 

 swim about in the water in clusters until they come in contact with 

 a female colony ; then they separate from each other, bore their w^ay 

 into the soft jelly of the female colony, and ' fertilize ' the egg-cell, 

 that is to say, each male cell fuses with a female cell and forms 

 a 'lasting spore,' exactly as in Pandorina. 



In Volvox the state of matters is similar to that in Eiulorina ; 

 here again, in addition to the ' asexual ' reproduction through the 

 'Parthenogonidia' which are like egg-cells in appearance (Fig. 63, y1, 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 diflferentiation in accord- 

 ance 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 eofcr 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 (Mobius). Fig. 64 shows 

 an ovum of this species surrounded by spermatozoa. 



In the course of the evolution of species this contrast between 

 female and male germ-cells became more and more marked, not 

 always in the same direction, however, but in one or another according 



