The sperm nucleus now increases in size, and its chromatin changes into a reticulate form. Sperm aster and sperm nucleus, the aster preceding, now move toward the egg nucleus. As the nuclei approach each other the sperm nucleus increases still more in size, until both become almost of the same size (Fig. C.). The chromatin network of each nucleus now breaks up into a number of chromosomes and the nuclei come into contact and fuse together. The centrosome, together with its aster, divide now into two parts, and the two daughter centrosomes move apart to the opposite poles of the ovum, and the typical amphi-aster of cell division, as above described, is formed (Fig. D.). The nuclear membranes now disappear and the chromosomes are drawn together into the equatorial plate where each splits longitudinally. The halves are drawn by the mantle fibrils toward the opposite poles where they are transformed into two daughter nuclei (Fig. E.). In the meantime the cytoplasm has also divided. The result are two new cells. This process of division is repeated continuously in each of the resulting generations of cells. From the mass of cells thus formed develops the new organism. CHAPTER VII THE FUNCTION OF THE MALE GENERATIVE ORGANS _Testicles._—The function of the testicles is threefold. There is first the inner secretion of certain substances, spermines, which are the cause of the male secondary characteristics. Eunuchs who have been operated upon before puberty show feminine traits in their appearance throughout their lives. They have sloping shoulders, flabby muscles, beardless faces, high-pitched, squeaky voices and festoons of fat on breasts and hips. Secondly, certain stimuli starting from the testicles serve to increase the tonus of the centres of erection. The main function of the testicles is the production of spermatozoa. [Illustration: CUT XXX. _Spermatogenesis._ 1, basement membrane; 2, Sertoli’s or sustentacular cell; 3, spermatogone; 4, spermatocytes; 5, spermatids; 6, heads of spermatozoa.] _Spermatogenesis._—The testicles belong to the class of tubulous glands. The tubules are through the greater part of their course convoluted, tubuli contorti, toward the end they become straight, tubuli recti. The wall of the convoluted tubules consists of three layers, (1) an external connective tissue layer, (2) a middle basement membrane, and (3) interiorly, an epithelial lining. The epithelium consists of two kinds of cells, first the supporting or the sustentacular or Sertoli’s cells and secondly the glandular cells.