36 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
the corium and then are coiled in order to obtain greater length. The 
acinous glands are represented by the sebaceous glands in connection 
with each hair (fig. 26, g), and by the scent glands in the anal or in- 
guinal region of many carnivores, rodents and edentates. Others — 
may occur in very diverse regions as on the face (bats, deer), in 
the occipital (camel) or temporal region (elephant) or on the legs 
(swine). 
The mammary or milk glands are now known to be modified tubu- 
lar glands possibly derived from sweat glands. In the monotremes the 
simplest condition is found, numbers of glands opening into a pair of 
sacs in the sides of the marsupium, or pouch where the young are kept, 
Fic. 29.—Scheme of different kinds of nipples, based on figures by Weber. Single 
line, ordinary integument, double line, that of primary mammary pocket. A, primitive 
condition, found in Echidna; B, human nipple; D, Didelphys before lactation; C, same at 
lactation; Z, embryonic, F adult condition in cow. Band C are true nipples, F a pseudo- 
nipple (teat). 
on the ventral side of the body. In the marsupials there is a slight nip- 
-ple developed from the bottom of the pocket. In the higher groups of 
mammals the first appearance of the milk glands is the formation of a 
‘milk line,’ a ridge on either side of the body from in front back to the 
inguinal region. This is soon divided into ‘milk points’ from each of 
which there is an ingrowth of epidermis into the corium, the interme- 
diate parts of the line disappearing. Each of the points may develop 
into a definitive mamma, but not all of them come to full development, 
for the number in the adult is less then in the embryo, varying from a 
single pair to eleven in Centetes, the number roughly corresponding to 
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