10. But since at some time or other conjugation is apparently necessary in order to restore vitality, the immortality at this point ceases to be an individual immortality, and becomes rather a joint or racial immortality. 11. The main thing in conjugation would appear to be that the two factors should be complementary to each other, however differentiated, so that in their union the whole race-life should be restored, and the Regeneration therefore be complete. 12. The special sex-differentiation called male and female depends on the separation of the active from the sessile qualities (and other qualities respectively related to each) into two great branches. 13. Since the female takes the sessile part she appears sometimes as the goal and object of conjugation, and the more important factor; but actual observation so far shows each factor, male and female, to be equally important. 14. In the fertilized ovum there are an equal number of chromosomes derived from each parent; and if the female provides the shrine in which the new development takes place, the male (centrosome) appears as the organizing genius of the process. 15. This process, by which a fertilized germ-cell divides and redivides, and so builds up a “body,” is quite similar to that by which a protozoön divides and redivides to form a numerous colony. 16. A ‘body’ indeed _is_ such a colony, coöperatively associated in definite form, of which all the millions of cells are practically continuous with the original fertilized germ, and one with it. 17. Every cell in such a body has apparently the same nuclear elements as the original cell, equally derived from both parents; but is differentiated so far as to be able to fulfil its special part in the body. 18. The process of division of these microscopic cells is strangely exact and complex; and the various elements of the nucleus seem to be themselves divided into two, on each occasion, with strange preciseness. 19. The constituent cells of each race of animals have always a certain number of nuclear threads or chromosomes—fixed for that particular race. 20. When, therefore, a sperm-cell and germ-cell unite, they each first extrude or expel half the number of their chromosomes, so that after union the joint cell is provided again with the precise number of chromosomes characteristic of the race. 21. The exact nature of these ‘maturation’ divisions and expulsions is far from clear; but it would seem that they are carried out in such a way as, while retaining always the basic elements of the Race, to secure a continual and endless sorting of these into new combinations. 22. These complex evolutions occurring, as described, in the interior of the most primitive cells, look as much like the last results of some far antecedent or invisible operations (of which we know nothing) as like the first commencement of the visible organic world with which we are acquainted. [Footnotes]