CENTROSOME 7 



or those which are not undergoing division, the centrosome may already 

 have divided into a double body or diplosome preparatory to the next 

 division of the cell (Fig. i). 



Centrosomes have been detected in many forms of resting cells, and 

 it is assumed by some authorities that the centrosome is an invariable 

 constituent of the cells of the higher vertebrates. According to this 

 opinion the centrosome may become inconspicuous but it never loses 

 its identity. Often they are found very close to the nuclear membrane, 

 which may be indented to accommodate them; and rarely, as in certain 

 cancer cells and in one form of the worm Ascaris, they have been reported 

 as within the nucleus. They may occur near 

 the free surface of certain cells, usually in the 

 form of diplosomes, as shown in cell a, Fig. 8. 

 Just above the diplosome, such cells may send 

 out contractile projections of protoplasm 

 (pseudopodia), with the activity of which the 

 diplosome may be in some way associated. 

 Pseudopodia, with an underlying diplosome, 

 have been observed in the columnar cells of 

 the human large intestine. In cell b of Fig. 8 

 there are four diplosomes, one of which lies 

 beneath the protoplasmic projections. It is 

 believed that the diplosomes may multiply 



by fission, and that thus they may give rise to the numerous motile 

 hairs, or cilia, which project from certain cells. Of these they form the 

 basal bodies (Fig. 8, c). In many gland cells the centrosome lies in the 

 midst of the protoplasm where the secretion accumulates. The discharge 

 of the secretion is accomplished by the contraction of the protoplasmic 

 strands in which the centrosome is lodged. In all these relations the 

 centrosome appears to be a center for motor activities, and it is described 

 as the kinetic or dynamic center of the cell. 



FIG. 8. CELLS OF THE EFFERENT 

 DUCTS OF THE TESTIS OF A 

 MOUSE. (After Puchs.) 



To show diplosomes, and (in c) 

 cilia with basal bodies. 



CELL WALL. 



The protoplasm at the surface of certain cells floating in the blood or 

 lymph forms a thin pellicle, apparently as a result of protoplasmic con- 

 centration, or other reaction to the surrounding medium. Cells which 

 line the greater part of the digestive tube, and have only one surface 

 directed toward the intestinal contents, are provided with a thick wall on 

 the exposed surface. Such a wall is called a cuticular border, or cuticula. 

 On the other sides of these cells, the membrane is much thinner, and on 

 the basal surface it is sometimes lacking. In such cases the protoplasm 

 appears to be continuous with that of the underlying cells. In other cases 



