INTEGUMENT. 35 
tion between scales and hairs, since in the mammals with scales the 
hairs are usually arranged in groups of three or five behind each scale 
(fig. 27); while in those without scales the hairs are frequently grouped 
in the same manner. The illustration (fig. 28) is interesting as 
showing the arrangement in man and the possible relation to ancestral 
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Fic. 27.—A, arrangement of the two kinds of hair in Ornithorhynchus; B, Arrangement 
of hair in Ptilocerus lori, with the probable relation of the hair to the ancestral scales; 
both after Meijere. 
Fic. 28.—Arrangement of the hairs in groups of threes and fives in the human embryo, 
with the probable ancestral arrangement of the scales; after Stohr. 
scales. The statement is also made that at first the hairs are arranged 
in longitudinal rows and that the grouping comes later. 
The mammalian skin is usually rich in glands which are of two 
types, tubular and acinous (p. 18). To the first belong the sweat glands, 
which extend from the Malpighian layer, where they arise, down through 
