134 Heredity and Environment 



germ cells and their development we should have little need for 

 theories. In the first chapter we looked at the germ cells and 

 their , development from the outside, as it were; let us now look 

 inside these cells and study their minuter structures and func- 

 tions. 



Only a beginning has been made in this minute study of the 

 germ cells and of their transformation into the developed animal, 

 and it seems probable that it may engage the attention of many 

 future generations of biologists, but nevertheless we have come 

 far since that day in 1875 when Oscar Hertwig first saw the ap- 

 proach and union of the egg and sperm nuclei within the fertilized 

 egg. Indeed so rapid has been the advance of knowledge in this 

 field that many of the pioneers in this work are still active in 

 research. 



I. Fertilization, a. Stimulus to Development. The development 

 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 and 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- 



