ELEMENTARY VITAL PHENOMENA 



169 



and stars in echinoderms and sponges, are formed within the cell 

 itself, and only after they have reached a certain size are they 

 given off to the outside by the customary mode of excretion of 

 solid bodies (Fig. 61). 



Amoeba shows best the mode of excretion of substances that lie 

 in the interior of the cell as solid masses. It has been seen that 



FIG. 59. Cross-section of bone. The 

 compact ground-substance lies 

 between the star-shiped bone- 

 cells. In the middle of the sec- 

 tion is a cross-section of a bone- 

 canal. (After Hatschek.) 



FIG. 60. ^Hyaline cartilage. Between the indi- 

 vidual cells a solid, hyaline ground-substance 

 has been excreted. (After Hatschek.) 



in the ingestion of food by Amoeba the food-ball enclosed in a 

 food-vacuole lies finally within the protoplasm. In this vacuole, 

 which may be termed a digestive vacuole, all digestible substance 

 becomes dissolved, and passes into the protoplasm ; but the indigest- 

 ible residue, such as shells of algae and of diatoms and the chitinous 

 masses of rotifers, remain in the vacuole, and become excreted 

 in the following manner : By the creeping of the Amoeba the 

 digestive vacuole in the streaming protoplasm comes to lie very 

 near the surface, so that its contents are separated from the 

 medium merely by a thin delicate wall of protoplasm. In such a 

 case the wall breaks very easily by the protoplasm flowing in all 





FIG. 61. Formation of a triradiate calcareous star in an echinoderm cell. (After Semon.) 



directions away from the thinnest place, and the contents of the 

 vacuole together with the solid mass are emptied to the outside 

 (Fig. 62). This mode of removal of solid constituents from the 

 protoplasm is found exclusively in cells that do not possess a 

 membrane, and hence chiefly in amoeboid cells of all kinds. 



A transition between the method of output of liquids and that 

 of solids is represented by the secretion of mucus. The mucous 



