3. The Origin of the Sex Cells
The sex cells are the latest of all cells of a developing organism to reach maturity, and yet they may be among the earliest to make their appearance. Every sex cell, like every other type of cell, is a lineal descendant of the fertilized egg (Fig. 22), but the period at which the sex cells become visibly different from other cells varies from the first cleavage of the egg in some species to a relatively advanced stage of development in others.
(a). The Division Period. Oogonia and Spermatogonia
When the primitive sex cells are first distinguishable they differ from other cells only in the fact that they are less differentiated; they have relatively larger nuclei and smaller cell bodies—a condition which is indicative of little differentiation of the cell body since the products of differentiation such as fibres, secretions, etc., swell the size of the cell body, but do not contribute to the growth of the nucleus. These primitive sex cells or gonia divide repeatedly, but the oogonia grow more rapidly and divide less frequently than the spermatogonia. As a result of this difference in the rate of growth and division the spermatogonia become much smaller and immensely more numerous than the oogonia. This period in the genesis of the sex cells is known as the division period (Fig. 22).
(b). The Growth Period. Oocytes and Spermatocytes
This period of rapid cell divisions is followed by a period of growth without division during which the developing sex cells are called primary oocytes or spermatocytes. This growth period may be very long in the case of the oocytes, lasting, for example, in the human female from the time of birth to the end of the reproductive period; during this long time the oocytes in the ovary probably never divide—there are as many of them at birth as at any later time; during this period of growth the ovarian egg becomes relatively large, in some animals, e.g., birds, the largest of all cells. The growth period of a spermatocyte lasts for a briefer time than does that of an oocyte so that the former remains relatively small (Fig. 22).
All of the cell divisions which take place during the division period are of the usual kind, in which every chromosome splits lengthwise into two and the two halves then separate and move to opposite poles of the spindle where they break up into threads and granules and form the daughter nuclei, as is shown in Fig. 24. But during the growth period of the oocytes and spermatocytes the chromosomes form a closely wound coil of long chromatin threads (Fig. 33), and when these threads uncoil later it is seen that the chromosomes have united in pairs (Figs. 33 D and E, 34 B, 35 B); this process is known as synapsis, or the conjuga-
