344 ~ A Lesson in Comparative Histology. [June, 
ureters of mammals (Englemann) frequently have two nuclei, and 
Dr. E. L. Mark, in his very valuable memoir on the Cuoccide, 
states that in the malpighian vessels of those insects he has like- 
wise found cells with two nuclei. 
The height of epithelial cells may be still further diminished, 
so that in some cases it may be said to have nearly disappeared, 
the cells assuming the form of a thin lamella. This is the case 
upon the-air tubes. If one of these be colored and mounted in 
the usual way, the flattened epithelial cells may be easily recog- 
nized by their oval nuclei, Fig. 3 6. Each nucleus contains one or 
sometimes two minute spherical 
dots, eccentrically placed; these 
are the nucleoli. We have now 
seen the three constituent parts, 
u which probably always enter into 
Fic. 3—Small air tube from the the composition of every cell; 
ae these are the protoplasm, the 
nucleus and the nucleolus. In addition we often find that the 
outside layer of the protoplasm becomes hardened and more 
resistent, and it is then called the membrane. 
In every epithelium we distinguish two kinds of surfaces on 
each cell, those which lie against other cells, and those which are 
free, facing the cavity. On the free surfaces the membrane 15 
often considerably thickened, and the thickened portions are then 
so joined together that they form a continuous lamella, which is 
called a cuticula. Now the flat epithelium of the air tubes forms 
a very curious cuticula, which lines all the trachee, and = 
remarkable for being thickened in some places more than ” 
others, thus developing a spiral thread, which can be seen in Fig. 
3, underneath the nuclei. The spiral filament was observed very 
long ago, but its real nature was only recently discovered. For 
a more detailed account the reader is referred to the NATURALIST 
for July, 1877. 
It is hoped that these illustrations will suffice to exemplify the 
more important features of epitheliums, tissues which are found 
in all animals except the protozoa, and represent one of the 
simplest and most frequent modes in which cells are aggregated. 
I propose to add a brief account of the structure of the digestive 
: ee ch 
canal, in order to show some of the further modifications whic a 
epitheliums may undergo. 
