Minot. ] 206 [April 2, 
‘ 
so that the distinction of the axes is lost, but in every true epithelium 
or endothelium, with but one layer of cells, the distinction is evident. 
4. Epitheliums increase their surface by the formation of depressions 
(invaginations), or of projecting folds (evaginations). This is a 
familiar principle, and therefure need not be further considered. 
5. Structural modifications of epitheliums usually affect similarly 
a whole cluster or tract of cells, but rarely isolated cells only. The 
illustrations of this principle may be found in every class of ani- 
mals —as for instance when one area or region of the epidermis is 
composed of higher cells than the rest — or again in the formation of 
the macula acustica in the otocysts of Heteropods and Vertebrates; 
glands often arise in this way, first a pit is formed, as in the peptic 
and mucous glands of vertebrate stomachs, and the little cluster of 
cells lining the bottom of the pit are differentiated into the gland 
cells. 7 
The instances in which isolated cells are differentiated into distinet 
forms are much rarer, but are nevertheless often important. Thus the 
lasso-cells of the Coelenterata are scattered singly, but show a ten- 
dency to form limited groups. The numerous unicellular glands 
should ke mentioned here, and also the scale-cells of Lepidoptera 
and the hair-cells of Orthoptera!. The most important and frequent 
case, since it is apparently common to all the Metazoa, is the method 
of forming the egg, by the modification of a single epithelial cell. 
On the other hand there are cases in which the differentiation of 
the isolated cells, which earlier authors have described, has been 
denied by more recent investigators. Thus it had been affirmed that 
the hair cells were scattered singly in the epithelium of the olfactory 
region, and of the auditory ampullae of vertebrates, but Exner as 
regards the first organ and Kuhn as regards the second have affirmed 
that the modification known as the hair cells affects clusters, i. e., 
the sensory area as a whole, not scattered cells. However this may 
be, it is not of great importance for us to know in connection with 
the present subject, for it is unquestionable that both forms of dif- 
ferentiation actually occur, and whether in a special instance one or 
the other mode obtains is a secondary problem. 
1 It is probably not generally known that each scale on the wings of moths is, 
according to the interesting observations of Semper, merely an outgrowth from a 
single isolated cell. I have reasons for thinking that each of the minute hairs, 
which surmount the pores of the cuticula in the grasshoppers, is the product of a 
single cell. 
