GERM CELLS AND THEIR FORMATION 



89 



containing a great deal of potential energy which becomes 

 kinetic after fertilization, during the early stages of develop- 

 ment. 



Upon examining the internal structure of the egg we find 

 that the nucleus is unusually large in most cases, spherical or 

 ovoid, and with or without a nuclear membrane (Figs. 43, 45). 

 It is located centrally or eccentrically, usually the latter if the 

 egg contains an appreciable amount of food material. The 

 chromatic network of the nucleus may be either dense, or so 

 open as to give, even after staining, the appearance of a lighter 



FIG. 45. Axial section through the oocyte of the Annulate, Nereis. After 

 Lillie, slightly modified, c, cortical protoplasmic layer (exoplasm) ; n, nucleus; 

 no, nucleolus; o, oil vacuoles; v, vitelline membrane; y, yolk bodies. 



area in the cytoplasm (Figs. 43, 45), often known as the germinal 

 vesicle. In some eggs the nucleus is in the process of division at 

 the time of egg-deposition (Figs. 42, 46); this condition will 

 be explained later. The actual morphological composition of 

 the nucleus is one of the chief characteristics of the egg, 

 and this too is reserved for consideration in Chapter IV. 

 There is commonly a large plasmosome or nucleolus of varying 

 form and size (Figs. 43, 45). Centrosome and centrosphere are 

 typically present at this time, though often minute and difficult 

 to observe; later these structures disappear entirely. Fre- 

 quently the cytoplasm contains unusual bodies termed "yolk- 

 nuclei." The term yolk-nucleus includes organs of several 

 different types, in some way related apparently to the formation 



