THE CELL ULAR BASIS OF HEREDITY 1 1 3 



certain external or internal conditions; in other cases, as Loeb dis- 

 covered, eggs which would never develop if left to themselves may be 

 experimentally stimulated by physical or chemical changes in the envi- 

 ronment, 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 partheno- 

 genesis 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 en- 

 vironmental 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 d}^es, 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 con- 

 densed 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 appeara 

 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 cur- 

 rent. Probably as a result of this current the sperm nucleus and centro- 

 some 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 



