HISTOLOGY OF THE FROG 81 



between adjoining cells, and these spaces, which communi- 

 cate with the lymph system, are generally traversed by 

 exceedingly fine protoplasmic fibrils passing from cell to cell, 

 and so establishing protoplasmic continuity throughout the 

 whole tissue. But, in spite of this protoplasmic continuity, 

 we recognise that the tissue is not a single tract of protoplasm 

 containing many nuclei, but is really composed of individual 

 cells, just as we should recognise that a house is really com- 

 posed of a number of rooms, though they all communicate 

 by means of doors. Columnar cells may be tall and thin, 

 or short and broad, or mid-way between these two extremes. 

 Their nuclei are usually placed mid-way between their free 

 ends and their bases. 



Such cells as those just described are simple columnar 

 epithelial ceils ; but there are many varieties of columnar 

 epithelium. One of the most common of these is ciliated 

 columnar epithelium, to be obtained from the frog's palate, or, 

 better, from the windpipe of higher vertebrates, or from the 

 gills of such a mollusc as the fresh-water mussel or the oyster. 



If a minute scraping be taken with the point of a knife 

 from the roof of the buccal cavity of a freshly-killed frog, 

 and the whitish material thus obtained be mounted on a 

 glass slip in a '75 solution of common salt and examined under 

 the microscope, it will be seen to consist of a number of trans- 

 lucent rounded cells, in most of which a large nucleus can 

 be seen. There are several kinds of these cells, but some of 

 them are conspicuous because of a continual shimmering 

 movement on one of their surfaces ; if such a cell be isolated 

 from the remainder of the mass it will be seen to gyrate and 

 spin round and round in the salt solution. After some time 

 the movement becomes slower, and then it can be seen that 

 the cell bears on one surface a tuft of very fine, hair-like 

 transparent processes of highly contractile protoplasm, called 

 cilia. When the movements have slowed down, it may be 

 seen that the cilia work in unison, with a sort of lashing 

 movement, bending very sharply in one direction, and then 

 more slowly relaxing till they regain their straightness, when 

 they again bend sharply as before. Tracts of these cells cover 

 the frog's palate, and their cilia, by their united action, drive 

 mucus and small particles of foreign matter along in a given 

 direction. If the ciliated cells are killed by a drop of very 



