HISTOLOGICAL DEMONSTRATIONS. 69 



next day, when this has dried, cover it with white zinc 

 cement (see 358). 



(This preparation will be required for fat cells and 

 areolar tissue on a future day.) 



88. Ciliated Epithelium. Ciliary Motion may be 

 readily studied in the gill of the mussel. The salt-water 

 mussel is much to be preferred to the fresh-water one. In 

 opening the shell, collect the salt water in a watch glass 

 for future use. The gill consists of parallel bars, the free 

 ends of which are at its free margin. 



a. With scissors and forceps remove a small piece of 

 the gill, including its free margin, place it on a slide, and 

 apply a drop of sea water. As the gill in each half of the 

 animal consists of a double membrane, separate the two 

 portions with a pair of needles (a dissecting microscope (Fig. 

 34) is here of service), and then very gently pull asunder a 

 few of the bars of one of the membranes ; apply the cover- 

 glass; place the preparation on the tin hot stage (Fig. 47), 

 and examine. 



(L.) Observe the bars with their free rounded ends 

 covered with cilia moving " like a field of corn before the 

 wind." Particles in the surrounding fluid are propelled 

 in a definite direction, the same as that in which the cilia 

 bend. 



(H.) The cilium tapers from base to tip. (When 

 motionless, and examined with a power of about 1000 

 diam., it is seen to be a filament, flat, perfectly trans- 

 parent, and apparently homogeneous.) The cells on 

 which the cilia are placed cannot be clearly recognised in 

 this preparation, but the clear band at the margin of the 

 cells, from which the cilia spring, may be distinctly seen. 

 The movement of the cilium has a definite direction, con- 

 sisting in its becoming alternately bent and straight. The 

 propulsion of the surrounding fluid in the direction of the 

 bend may be accounted for by supposing that the velocity 

 with which the cilium bends is greater than that with which 

 it regains its erect position. 



b. Effect of heat. The hot stage is already under the 

 slide, apply heat as in the experiment with Tradescantia 



