NEPHRIDIAL TISSUES 



355 



contains distinct chromatin granules of nearly uniform size (Fig. 321, 

 A, B). The excretion product first appears as a small spherical mass 

 of dense homogeneous sub- 

 stance, lying within the cyto- 

 plasm near the nucleus. This 

 body continues to increase in 

 size until it has become a 

 large, dense sphere twelve 

 microns or more in diameter. 

 About this sphere the cyto- 

 plasm is applied as a thin film 

 (Fig. 321, Z)). The nucleus 

 is distorted and crowded to 

 one side inclosed in a small 

 amount of cytoplasm. The 

 chromatin of the nucleus has 

 become less distinct. The 

 excretion or concretion sphere 

 now breaks up into small 

 bodies, which are scattered 

 throughout the cell that has 

 somewhat enlarged (Fig. 321, 

 E and F). In the meantime, 

 the nucleus shows a marked 

 tendency to divide, and fre- 

 quently amitosis is effected. 

 The nucleus or nuclei finally 

 disintegrate, and the cell has 

 completed its course of excre- 

 tory activity. The cells in this 

 condition or their fragments leave the gland through the reno-pericar- 

 dial duct to the kidney, and from thence to the exterior. 



The chloragogenic cells of Lumbricus are found forming columnar 



FIG. 320. Epithelium from the coelomic cavity of 

 Lumbricus. A , vertical section through the epithe- 

 lium with some of the underlying connective tissue, 

 muscle, and blood vessels ; _B, surface view of three 

 cells and parts of others; C, three wandering cells 

 in the coelomic cavity; w.c., wandering cells; ex.c., 

 excretory cells. X 1300. 



FIG. 321. Six stages of secretion elaboration by the amcebocytes or " wan- 

 dering cells" from the pericardial gland of Unio. X 870. 



epithelium upon coelomic surfaces. They are most abundant about the 

 large blood vessels and the intestine. The typhlasole is filled with a 



