172 



THE CELL 



that the new particles become deposited in definite positions, in definite 

 quantities, and in definite directions, between those already present." 



Moreover, the process of intussusception is not to be disre- 

 garded in those cases where calcium and silicon salts are deposited 

 in the cell-wall, for this mostly occurs at a later period, the salts 

 being frequently only found in the superficial layers. It could 

 only be proved that it is impossible for particles of cellulose to be 

 deposited in a similar manner, if it could be shown that cellulose 

 is actually only produced by the direct metamorphosis of layers 

 of protoplasm. However, up till now this is anything but proved; 

 and, moreover, it seems that the study of plant anatomy, by means 

 of microscopic observation alone, is insufficient to establish this 

 theory, and that in addition a very much improved and advanced 

 knowledge of micro-chemistry must be reached, as in the case 

 mentioned on pp. 153, 154. Consideration of the statements made 

 there shows especially, that under certain conditions in the for- 

 mation of cellulose there is not the marked difference that is 

 frequently considered to exist between growth by apposition and 

 growth by intussusception. 



Cuticular structures are the skin-like formations with which a 

 cell covers its external surface not all over, however, but only 

 on one side. In the animal kingdom, those cells which are situated 

 on the surface of the body, or which cover the internal surface of 

 the alimentary canal, are frequently provided with a cuticle, which 

 protects the underlying protoplasm from the hurtful influences 

 of the surrounding media. The cuticle usually consists of thin 

 lamellae, intersected by fine parallel pores, into which delicate 

 processes stretch from the underlying protoplasm. As cuticular 

 formations of a peculiar kind, which exhibit at the same time a 

 very marked structure, the outer portions of the rods and cones 

 in the retina may be cited. 



Cuticular membrane-like formations, consisting of cells united 



Fig. 73. Epithelium with cuticle of a Saw-fly {Cimbex coronatus) (from R. Hertwig ; 

 Fig. 21 J): c cuticle; e epithelium. 



