THE VITAL PROPERTIES OF THE CELL 



159 



Under such circumstances the cells are filled with drops of oil 

 (oily seeds), with granules of various albuminous substances 

 (vitellin, gluten, aleuron), with crystalloids of proteinaceous sub- 

 stance, or with starch granules, about which more will be said 

 later. 



The above-mentioned internal plasmic pi-oducts being only tem- 

 porarily accumulated during metabolism before being utilised, 

 vary considerably in composition, but there are others which 

 attain a higher degree of organisation, and which participate 

 permanently in the functions of the cell. To such belong the 

 internal skeletal structures of the protoplasm, the various sub- 

 stances in plant cells, described under the common name of 

 trophoplasts, the cnidoblasts of Coelenterata, and, finally, the 

 sheaths of the muscle and nerve fibres, etc. 

 Internal skeletons 



are found in the bodies 



of a large number of 



Protozoa, but especi- 

 ally in great vainety 



and beauty in Radio- 



larians. They consist 



sometimes of regularly 



arranged spicules, 



sometimes of a fine, 



open trellis-work, and 



sometimes of a com- 

 bination of the two 



kindsof structures (Fig. 



66). In some families 

 of Radiolarians they 



FIG. 66. Hnlhmmi trinnceus (from R. Herlwiz, Zool., 

 Fig. 82) : a external, i internal trellis work ; cfc central 

 capsule- u-fc soft extra capsular body; 71 internal vesicle 



are composed of an or- (n u cleu ^. 

 ganic substance which 



is soluble in acids and alkalies, but in most cases, on the contrary, 

 they consist of silicious material which is united to an organic 

 substratum, just as, in the bones of Vertebrates, the phosphates 

 are united with the ossein. In each species the skeleton has a 

 constant and characteristic structure, and follows certain fixed 

 laws during the process of its development (Richard Hertwig, 

 5, 40). 



Under the name trophoplasts, the highly organised difl 

 tiated products of vegetable protoplasm are included ; these occur 



