THE VITAL PROPERTIES OF THE CELL 157 



contractile vacuoles or reservoirs (p. 85). This occurs with 

 especial frequency in Ciliata. 



Finally, it occasionally, although rarely, happens that the sap 

 collects into special vacuoles ; this may occur in various kinds of 

 animal cells, and especially in structures which have a supporting 

 function in the body. In the tentacles of many Coelenterates, in 

 certain appendages of Annelids, and also in the chorda dorsalis of 

 Vertebrates, there are comparatively large vesicular cells, which 

 are separated from the exterior by a thick membrane, and which 

 contain hardly anything but cell sap, only a very minute quantity 

 of protoplasm being present. This is spread out in a very thin 

 layer over the membrane, extending threads here and there across 

 the sap space ; the nucleus is generally embedded in a somewhat 

 denser collection of protoplasm, either in the peripheral layer, or 

 in the network. Here also, as in plants, the firm cell-wall is 

 tensely distended in consequence of the osmotic action of the 

 substances in the sap. Although no experimental investigations 

 have yet been made concerning the turgescence of the organs in 

 question, yet it can only be explained in this manner: that the 

 notochord functions in the body of a Vertebrate as a supporting 

 organ. The very numerous small turgescent notochord cells 

 being built up into one organ, and also shut off from the exterior 

 by means of a firm elastic sheath, their individual tensions are 

 summed up, and through the internal pressure of the sheath the 

 structure is kept rigid. 



The absorption and secretion of sap occur in nuclear substance, 

 just as in protoplasm. The sap serves the same purpose in both 

 cases, namely to offer a large surface to the active substances, 

 and to put them into direct communicabion with the nutrient 

 fluid. 



Although the formation of sap vacuoles occurs but rarely in 

 animal cells, various substances, such as fat, glycogen, mucin, 

 albuminates, etc., frequently separate out from the protoplasm. 



The fat is seen to occur at first as small drops in the proto- 

 plasmic body, resembling the drops of cell sap in young plant 

 cells. Just like such vacuoles, the droplets increase in size, and 

 run together, producing, finally, one single large drop, which fills 

 the whole internal space of the cell, and which is surrounded by 

 a delicate cell -membrane, and by a thin layer of protoplasm, 

 which contains the nucleus. 



Glycogen collects in separate particles in the liver cells ; these 



