86 GENERAL EMBRYOLOGY 



this chapter we shall first describe the form and structure of the 

 typical germ cells of the Metazoa, as they appear when fully 

 formed and ready to function, that is, just prior to fertilization. 

 We shall then mention some of the more important modifica- 

 tions of structure shown in different groups, and finally give a 

 brief account of the formation and history of the germ cells up 

 to the time when they are ready actually to enter upon the real 

 process of development. The details of certain phases in the 

 history of the germ-cell nuclei, namely, the maturation processes, 

 are of such importance that we shall refer to them only briefly 

 in this chapter and devote that following to a more extended 

 account of these events. 



Among the Metazoa the fully formed germ cells are always 

 of two very unlike types, the ova or eggs and the spermatozoa 

 or sperm cells. These cells are alike only with respect to their 

 nuclear structure and composition. Their form differences 

 are associated with fundamental differences in function. The 

 egg cell contains by far the greater share of the substance which 

 is to form the material basis of the new individual. The sperm, 

 on the other hand, contributes little substance, and that chiefly 

 nuclear, to the new individual. One of its important functions 

 seems to be that of ensuring to the comparatively passive egg, 

 a stimulus which leads it to react, i.e., to commence develop- 

 ment; and the sperm's nuclear configuration, with that of the 

 egg, together appear to determine the course of development 

 to a large extent, if not wholly. 



The substance of which the ovum is composed is not a homo- 

 geneous protoplasm. The cytoplasm is differentiated and 

 organized into a definite structural and chemical (energetic) 

 configuration. The details of this configuration are uniform 

 in the eggs of each animal kind, i.e., it is specific. This cy to- 

 pi asmic structure of the ovum, although itself apparently 

 determined primarily by nuclear activity, is of great importance 

 in maintaining the continuity and uniformity of organismal 

 characteristics through successive generations (heredity). 



The ova are less modified in external form than the sperma- 

 tozoa, and often approach the form of a typical cell, except 



