22 



COLLEGE ZOOLOGY 



Figure 7. Electron micrograph of a cross section of a sea urchin egg, magnification 15,600 

 times. Note the nucleus (N) with the dark nucleolus (Nc) to the left; it has been demonstrated 

 that the dark wavy nuclear membrane (M) has "holes" in it, covered with a thin membrane. 

 The yolk granule (Y) barely visible under a light microscope is seen plainly, and the protoplasmic 

 reticulum (R) is well shown. (Electron micrograph courtesy of B.A. Afzelius, reprinted by 

 permission of Experimental Cell Research, 8:155, 1955, and Academic Press Inc., New York.) 



bounded by a nuclear membrane. Within 

 the nucleus is a colorless fluid, the nuclear 

 sap (nucleoplasm), in which there is a sub- 

 stance that has a strong afhnity for certain 

 dyes; this is known as chromatin. Some nu- 

 clei contain an intensely staining, spherical 

 body, the nucleolus. 



Various types of bodies may occur in the 

 cytoplasm. Often near the nucleus is lo- 



cated a specialized portion of the proto- 

 plasm, the centrosphere, in the center of 

 which are one or two deeply staining bodies, 

 the centrioles. The term centrosome in- 

 cludes the centrosphere together with the 

 centrioles. Spherical vesicles of liquid of var- 

 ious sizes, called vacuoles, may or may not 

 be present. Spherical or rod-shaped mito- 

 chondria contain enzymes which are in- 



