certain external or internal conditions; in other cases, as Loeb discovered, eggs which would never develop if left to themselves may be experimentally stimulated by physical or chemical changes in the environment, so that they undergo regular development. The development of an egg without previous fertilization is known as parthenogenesis or virgin reproduction; if it occurs in nature it is natural parthenogenesis, if in experiments it is artificial parthenogenesis. Natural parthenogenesis is relatively rare and in the vast majority of animals and plants the egg does not begin to develop until a spermatozoon has entered it.
But the spermatozoon not only stimulates the egg to develop, as environmental conditions may also do, but it carries into the egg living substances which are of great significance in heredity. Usually only the head of the spermatozoon enters the egg (Figs. 4-7) and this consists almost entirely of nuclear material which has a strong chemical affinity for certain dyes, and hence is called chromatin (Figs. 23 A and B); when the egg has matured and is ready to be fertilized its nucleus also consists of a small mass of chromatin (Fig. 23 C). Both of these condensed chromatic nuclei then grow in size and become less chromatic by absorbing from the egg a substance which is not easily stained by dyes and hence is called achromatin (Fig. 23 D and E). The chromatin then becomes scattered through each nucleus in the form of granules or threads which are embedded in the achromatin; this is the condition of a typical "resting" nucleus. The spermatozoon also brings into the egg a centrosome, or division center, around which an aster appears consisting of radiating lines in the protoplasm of the egg (Fig 23 B).
The moment that the spermatozoon touches the surface of the egg the latter throws out at the point touched a prominence, or reception cone (Fig. 4), and as soon as the head of the sperm has entered this cone some of the superficial protoplasm of the egg flows to this point and then turns into the interior of the egg in a kind of vortex current. Probably as a result of this current the sperm nucleus and centrosome are carried deeper into the egg and finally are brought near to the egg nucleus (Fig. 23 D and E). In the movements of egg and sperm nuclei toward each other it is evident that they are passively carried about by currents in the cytoplasm; the entrance of the sperm serves as a stimulus to the egg cytoplasm which moves according to its pre-established organization.
2. Cleavage and Differentiation
When the sperm nucleus has come close to the egg nucleus the sperm centrosome usually divides into two minute granules, the daughter centrosomes, which move apart forming a spindle with the centrosomes at its poles and with astral radiations running out from these into the cytoplasm (Fig. 23 F). At the same time the chromatin granules and threads in the egg and sperm nuclei run together into a smooth thick
