The Cellular Basis 133 



last analysis the causes of heredity and development are problems 

 of cell structures and functions, problems of the formation of 

 particular kinds of germ cells, of the fusion of these cells in fer- 

 tilization, and of the subsequent formation of the various types 

 of somatic cells from the fertilized egg cell. 



B. THE GERM CELLS 



Observations and experiments on developed animals and plants 

 have furnished us with a knowledge of the finished products of 

 inheritance, but the actual stages and causes of inheritance, the 

 real mechanisms of heredity, are to be found only in a study of 

 the germ cells and their development. Although many phenomena 

 of inheritance have been discovered in the absence of any definite 

 knowledge of the mechanism of heredity, a scientific explanation 

 of these phenomena must wait upon the knowledge of their causes. 

 In the absence of such knowledge it has been necessary to formu- 

 late theories of heredity to account for the facts, but these theo- 

 ries are only temporary scaffolding to bridge the gaps in our 

 knowledge, and if we knew all that could be known about the 

 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 



