134 Heredity and Environment 



of the individual may be said to begin with the fertilization of the 

 egg, though it is evident that both egg and sperm must have had 

 a more remote beginning, and that they also have undergone a 

 process of development by which their peculiar characteristics of 

 structure and function have arisen, a subject to which we shall 

 return later. But the developmental processes which lead to the 

 formation of fully developed ova arid spermatozoa come to a full 

 stop before fertilization and they do not usually begin again 

 until a spermatozoon has entered an ovum, or until the latter has 

 been stimulated by some other outside means. 



Parthenogenesis. In some animals and plants, eggs may de- 

 velop regularly without fertilization, the stimulus to development 

 being supplied by 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 regu- 

 lar 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 experi- 

 ments it is artificial parthenogenesis. Natural parthenogensis 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. 



b. Union of Gennplasms. But the spermatozoon not only stim- 

 ulates 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 spermato- 

 zoon enters the egg (Fig. 4) and this consists almost entirely of 

 nuclear chromatin (Fig. 4 D-H, 42 A-B) ; when the egg has ma- 

 tured and is ready to be fertilized its nucleus also consists of a 

 small mass of chromatin (Fig- 42 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 (Figs. 4 I-L, 42 D-E). 

 The chromatin then appears to become scattered through each 



