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 endoderm and mesoderm in shaded circles. The whole course of spermatogenesis and oogenesis is shown in the lower right of the figure beginning with the primitive 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.
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 undergone a process of development by which their peculiar characteristics of
