TEE CELLULAR BASIS OF HEREDITY 



in 



the germ cells and their development we should have little need of 

 theories. In the first lecture 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 functions. 



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 



Fig. 22. Diagram Showing 

 tub " Cell Lineage " of the 

 Body Cells and Germ Cells 

 in a Worm or Mollusk. The 

 lineage of the germ cells (" germ 

 track ") is shown in black, of 

 ectoderm in white, and of endo- 

 derm and mesoderm in shaded 

 circles. The whole course of 

 spermatogenesis and oogenesis is 

 shown in the lower right of the 

 figure beginning with the primi- 

 tive sex cells (Prim. Sex Cells) 

 and ending with the gametes, 

 the genesis of the spermatozoa 

 being shown on the left and of 

 the ova on the right. 



Gametes 



generations of biologists, but nevertheless we have come far since that 

 day, only about thirty-five years ago, when Oscar Hertwig first saw the 

 approach 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. 



1. Fertilization 



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 under- 

 gone a process of development by which their peculiar characteristics of 



